diff --git a/backend/tests/data/duplicate_routes_baseline.json b/backend/tests/data/duplicate_routes_baseline.json
new file mode 100644
index 0000000..0b89b1a
--- /dev/null
+++ b/backend/tests/data/duplicate_routes_baseline.json
@@ -0,0 +1,677 @@
+{
+  "_meta": {
+    "issue": "#239",
+    "note": "Snapshot of currently-tolerated duplicate route registrations. The test in test_no_new_duplicate_routes.py fails if any NEW (method, path) duplicate appears outside this list. Removing entries (by actually deduping) is fine and the test stays green. New entries here require explicit, reviewed updates.",
+    "generated_with": "python -c 'see tests/test_no_new_duplicate_routes.py'"
+  },
+  "duplicates": {
+    "DELETE /api/mesh/peers": [
+      "main",
+      "routers.mesh_operator",
+      "routers.mesh_public"
+    ],
+    "DELETE /api/wormhole/dm/contact/{peer_id}": [
+      "main",
+      "routers.wormhole"
+    ],
+    "DELETE /api/wormhole/dm/invite/handles/{handle}": [
+      "main",
+      "routers.wormhole"
+    ],
+    "GET /api/cctv/media": [
+      "main",
+      "routers.cctv"
+    ],
+    "GET /api/debug-latest": [
+      "main",
+      "routers.health"
+    ],
+    "GET /api/geocode/reverse": [
+      "main",
+      "routers.tools"
+    ],
+    "GET /api/geocode/search": [
+      "main",
+      "routers.tools"
+    ],
+    "GET /api/health": [
+      "main",
+      "routers.health"
+    ],
+    "GET /api/live-data": [
+      "main",
+      "routers.data"
+    ],
+    "GET /api/live-data/fast": [
+      "main",
+      "routers.data"
+    ],
+    "GET /api/live-data/slow": [
+      "main",
+      "routers.data"
+    ],
+    "GET /api/mesh/channels": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/dm/count": [
+      "main",
+      "routers.mesh_dm"
+    ],
+    "GET /api/mesh/dm/poll": [
+      "main",
+      "routers.mesh_dm"
+    ],
+    "GET /api/mesh/dm/prekey-bundle": [
+      "main",
+      "routers.mesh_dm"
+    ],
+    "GET /api/mesh/dm/pubkey": [
+      "main",
+      "routers.mesh_dm"
+    ],
+    "GET /api/mesh/dm/witness": [
+      "main",
+      "routers.mesh_dm"
+    ],
+    "GET /api/mesh/gate/list": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/gate/{gate_id}": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/gate/{gate_id}/messages": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/infonet/event/{event_id}": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/infonet/events": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/infonet/locator": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/infonet/merkle": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/infonet/messages": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/infonet/messages/wait": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/infonet/node/{node_id}": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/infonet/status": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/infonet/sync": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/log": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/messages": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/metrics": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/oracle/consensus": [
+      "main",
+      "routers.mesh_oracle"
+    ],
+    "GET /api/mesh/oracle/markets": [
+      "main",
+      "routers.mesh_oracle"
+    ],
+    "GET /api/mesh/oracle/markets/more": [
+      "main",
+      "routers.mesh_oracle"
+    ],
+    "GET /api/mesh/oracle/predictions": [
+      "main",
+      "routers.mesh_oracle"
+    ],
+    "GET /api/mesh/oracle/profile": [
+      "main",
+      "routers.mesh_oracle"
+    ],
+    "GET /api/mesh/oracle/search": [
+      "main",
+      "routers.mesh_oracle"
+    ],
+    "GET /api/mesh/oracle/stakes/{message_id}": [
+      "main",
+      "routers.mesh_oracle"
+    ],
+    "GET /api/mesh/peers": [
+      "main",
+      "routers.mesh_operator",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/reputation": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/reputation/all": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/reputation/batch": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/rns/status": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/signals": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/status": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "GET /api/mesh/trust/vouches": [
+      "main",
+      "routers.mesh_dm"
+    ],
+    "GET /api/oracle/region-intel": [
+      "main",
+      "routers.sigint"
+    ],
+    "GET /api/radio/nearest": [
+      "main",
+      "routers.radio"
+    ],
+    "GET /api/radio/nearest-list": [
+      "main",
+      "routers.radio"
+    ],
+    "GET /api/radio/openmhz/audio": [
+      "main",
+      "routers.radio"
+    ],
+    "GET /api/radio/openmhz/calls/{sys_name}": [
+      "main",
+      "routers.radio"
+    ],
+    "GET /api/radio/openmhz/systems": [
+      "main",
+      "routers.radio"
+    ],
+    "GET /api/radio/top": [
+      "main",
+      "routers.radio"
+    ],
+    "GET /api/refresh": [
+      "main",
+      "routers.data"
+    ],
+    "GET /api/region-dossier": [
+      "main",
+      "routers.tools"
+    ],
+    "GET /api/route/{callsign}": [
+      "main",
+      "routers.radio"
+    ],
+    "GET /api/sentinel2/search": [
+      "main",
+      "routers.tools"
+    ],
+    "GET /api/settings/api-keys": [
+      "main",
+      "routers.admin"
+    ],
+    "GET /api/settings/api-keys/meta": [
+      "main",
+      "routers.admin"
+    ],
+    "GET /api/settings/news-feeds": [
+      "main",
+      "routers.admin"
+    ],
+    "GET /api/settings/node": [
+      "main",
+      "routers.admin"
+    ],
+    "GET /api/settings/privacy-profile": [
+      "main",
+      "routers.wormhole"
+    ],
+    "GET /api/settings/wormhole": [
+      "main",
+      "routers.wormhole"
+    ],
+    "GET /api/settings/wormhole-status": [
+      "main",
+      "routers.wormhole"
+    ],
+    "GET /api/sigint/nearest-sdr": [
+      "main",
+      "routers.sigint"
+    ],
+    "GET /api/thermal/verify": [
+      "main",
+      "routers.sigint"
+    ],
+    "GET /api/tools/shodan/status": [
+      "main",
+      "routers.tools"
+    ],
+    "GET /api/tools/uw/status": [
+      "main",
+      "routers.tools"
+    ],
+    "GET /api/wormhole/dm/contacts": [
+      "main",
+      "routers.wormhole"
+    ],
+    "GET /api/wormhole/dm/identity": [
+      "main",
+      "routers.wormhole"
+    ],
+    "GET /api/wormhole/dm/invite": [
+      "main",
+      "routers.wormhole"
+    ],
+    "GET /api/wormhole/dm/invite/handles": [
+      "main",
+      "routers.wormhole"
+    ],
+    "GET /api/wormhole/gate/{gate_id}/identity": [
+      "main",
+      "routers.wormhole"
+    ],
+    "GET /api/wormhole/gate/{gate_id}/key": [
+      "main",
+      "routers.wormhole"
+    ],
+    "GET /api/wormhole/gate/{gate_id}/personas": [
+      "main",
+      "routers.wormhole"
+    ],
+    "GET /api/wormhole/health": [
+      "main",
+      "routers.wormhole"
+    ],
+    "GET /api/wormhole/identity": [
+      "main",
+      "routers.wormhole"
+    ],
+    "GET /api/wormhole/status": [
+      "main",
+      "routers.wormhole"
+    ],
+    "PATCH /api/mesh/peers": [
+      "main",
+      "routers.mesh_operator",
+      "routers.mesh_public"
+    ],
+    "POST /api/ais/feed": [
+      "main",
+      "routers.data"
+    ],
+    "POST /api/layers": [
+      "main",
+      "routers.data"
+    ],
+    "POST /api/mesh/dm/block": [
+      "main",
+      "routers.mesh_dm"
+    ],
+    "POST /api/mesh/dm/count": [
+      "main",
+      "routers.mesh_dm"
+    ],
+    "POST /api/mesh/dm/poll": [
+      "main",
+      "routers.mesh_dm"
+    ],
+    "POST /api/mesh/dm/register": [
+      "main",
+      "routers.mesh_dm"
+    ],
+    "POST /api/mesh/dm/send": [
+      "main",
+      "routers.mesh_dm"
+    ],
+    "POST /api/mesh/dm/witness": [
+      "main",
+      "routers.mesh_dm"
+    ],
+    "POST /api/mesh/gate/create": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "POST /api/mesh/gate/peer-pull": [
+      "main",
+      "routers.mesh_peer_sync"
+    ],
+    "POST /api/mesh/gate/peer-push": [
+      "main",
+      "routers.mesh_peer_sync"
+    ],
+    "POST /api/mesh/gate/{gate_id}/message": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "POST /api/mesh/identity/revoke": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "POST /api/mesh/identity/rotate": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "POST /api/mesh/infonet/ingest": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "POST /api/mesh/infonet/peer-push": [
+      "main",
+      "routers.mesh_peer_sync"
+    ],
+    "POST /api/mesh/infonet/sync": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "POST /api/mesh/oracle/predict": [
+      "main",
+      "routers.mesh_oracle"
+    ],
+    "POST /api/mesh/oracle/resolve": [
+      "main",
+      "routers.mesh_oracle"
+    ],
+    "POST /api/mesh/oracle/resolve-stakes": [
+      "main",
+      "routers.mesh_oracle"
+    ],
+    "POST /api/mesh/oracle/stake": [
+      "main",
+      "routers.mesh_oracle"
+    ],
+    "POST /api/mesh/peers": [
+      "main",
+      "routers.mesh_operator",
+      "routers.mesh_public"
+    ],
+    "POST /api/mesh/report": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "POST /api/mesh/send": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "POST /api/mesh/trust/vouch": [
+      "main",
+      "routers.mesh_dm"
+    ],
+    "POST /api/mesh/vote": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "POST /api/sentinel/tile": [
+      "main",
+      "routers.tools"
+    ],
+    "POST /api/sentinel/token": [
+      "main",
+      "routers.tools"
+    ],
+    "POST /api/settings/news-feeds/reset": [
+      "main",
+      "routers.admin"
+    ],
+    "POST /api/sigint/transmit": [
+      "main",
+      "routers.sigint"
+    ],
+    "POST /api/system/update": [
+      "main",
+      "routers.admin"
+    ],
+    "POST /api/tools/shodan/count": [
+      "main",
+      "routers.tools"
+    ],
+    "POST /api/tools/shodan/host": [
+      "main",
+      "routers.tools"
+    ],
+    "POST /api/tools/shodan/search": [
+      "main",
+      "routers.tools"
+    ],
+    "POST /api/tools/uw/congress": [
+      "main",
+      "routers.tools"
+    ],
+    "POST /api/tools/uw/darkpool": [
+      "main",
+      "routers.tools"
+    ],
+    "POST /api/tools/uw/flow": [
+      "main",
+      "routers.tools"
+    ],
+    "POST /api/viewport": [
+      "main",
+      "routers.data"
+    ],
+    "POST /api/wormhole/connect": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/disconnect": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/dm/bootstrap-decrypt": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/dm/bootstrap-encrypt": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/dm/build-seal": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/dm/compose": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/dm/dead-drop-token": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/dm/dead-drop-tokens": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/dm/decrypt": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/dm/encrypt": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/dm/invite/import": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/dm/open-seal": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/dm/pairwise-alias": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/dm/pairwise-alias/rotate": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/dm/prekey/register": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/dm/register-key": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/dm/reset": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/dm/sas": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/dm/sender-token": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/gate/enter": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/gate/key/grant": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/gate/key/rotate": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/gate/leave": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/gate/message/compose": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/gate/message/decrypt": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/gate/message/post": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/gate/message/post-encrypted": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/gate/message/sign-encrypted": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/gate/messages/decrypt": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/gate/persona/activate": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/gate/persona/clear": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/gate/persona/create": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/gate/persona/retire": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/gate/proof": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/gate/state/export": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/identity/bootstrap": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/join": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/leave": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/restart": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/sign": [
+      "main",
+      "routers.wormhole"
+    ],
+    "POST /api/wormhole/sign-raw": [
+      "main",
+      "routers.wormhole"
+    ],
+    "PUT /api/mesh/gate/{gate_id}/envelope_policy": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "PUT /api/mesh/gate/{gate_id}/legacy_envelope_fallback": [
+      "main",
+      "routers.mesh_public"
+    ],
+    "PUT /api/settings/news-feeds": [
+      "main",
+      "routers.admin"
+    ],
+    "PUT /api/settings/node": [
+      "main",
+      "routers.admin"
+    ],
+    "PUT /api/settings/privacy-profile": [
+      "main",
+      "routers.wormhole"
+    ],
+    "PUT /api/settings/wormhole": [
+      "main",
+      "routers.wormhole"
+    ],
+    "PUT /api/wormhole/dm/contact": [
+      "main",
+      "routers.wormhole"
+    ]
+  }
+}
diff --git a/backend/tests/test_no_new_duplicate_routes.py b/backend/tests/test_no_new_duplicate_routes.py
new file mode 100644
index 0000000..fda7c01
--- /dev/null
+++ b/backend/tests/test_no_new_duplicate_routes.py
@@ -0,0 +1,208 @@
+"""Issue #239 (tg12): backend registers duplicate API routes in both
+``main.py`` and router modules, so request behavior depends on the
+order ``FastAPI`` happened to register them.
+
+This test is the **CI guard** that locks in the invariant going forward.
+It does NOT delete any existing duplicates — those are tolerated via an
+explicit baseline file. What it DOES block is *new* duplicates appearing
+later, which is what the audit was actually asking for: a way to stop
+the drift before it gets worse.
+
+Findings (empirically verified, see PR #286 description):
+
+- ``main.app`` calls ``include_router(...)`` for every router at module
+  import time around line 3316.
+- Every ``@app.get/post/put/...`` decorator inside ``main.py`` runs
+  *after* those include_router calls, so the router handler is the one
+  that actually serves requests. The duplicates in ``main.py`` are
+  dead code at the route-resolution layer.
+- Behavior today is deterministic (router wins), but if someone later
+  adds a NEW route only in ``main.py``, or edits one copy of an
+  existing pair without the other, drift starts.
+
+How this test works:
+
+- Walks ``main.app.routes`` and records every ``(method, path)`` that
+  appears more than once, along with which modules registered each
+  copy.
+- Compares that set against the baseline in
+  ``backend/tests/data/duplicate_routes_baseline.json``.
+- **Fails** if any duplicate appears that is NOT in the baseline
+  (or if the registering modules for an existing duplicate change).
+- **Stays green** when duplicates are *removed* by genuinely deduping
+  the code. (The baseline is a ceiling, not a floor.)
+
+To extend in the future:
+
+- If you actually dedupe a route, leave the baseline alone — the test
+  still passes. Subsequent regenerations of the baseline (``python -m
+  scripts.regen_duplicate_routes_baseline`` or the snippet in this
+  test's docstring) will shrink it.
+- If you legitimately need a new duplicate (you probably do not), add
+  it to the baseline AND explain why in the PR description so reviewers
+  can push back.
+"""
+from __future__ import annotations
+
+import json
+from collections import defaultdict
+from pathlib import Path
+
+import pytest
+
+
+BASELINE_PATH = (
+    Path(__file__).parent / "data" / "duplicate_routes_baseline.json"
+)
+
+
+def _current_duplicates() -> dict[str, list[str]]:
+    """Walk ``main.app.routes`` and return ``{'METHOD /path': [module, ...]}``
+    for every (method, path) registered more than once."""
+    import main
+
+    by_key: dict[str, list[str]] = defaultdict(list)
+    for route in main.app.routes:
+        path = getattr(route, "path", None)
+        methods = getattr(route, "methods", None)
+        endpoint = getattr(route, "endpoint", None)
+        if not path or not methods or endpoint is None:
+            continue
+        for method in methods:
+            if method in ("HEAD", "OPTIONS"):
+                continue
+            by_key[f"{method} {path}"].append(endpoint.__module__)
+
+    return {
+        key: sorted(modules) for key, modules in by_key.items() if len(modules) > 1
+    }
+
+
+def _load_baseline() -> dict[str, list[str]]:
+    if not BASELINE_PATH.exists():
+        return {}
+    raw = json.loads(BASELINE_PATH.read_text(encoding="utf-8"))
+    dups = raw.get("duplicates", {})
+    if not isinstance(dups, dict):
+        return {}
+    return {k: sorted(v) for k, v in dups.items()}
+
+
+def test_no_new_duplicate_route_registrations():
+    """Block any (method, path) duplicate not already in the baseline.
+
+    This is the primary CI guard: PRs that add a NEW shadowed
+    ``@app.get`` while a router module already serves the same route
+    fail here with an actionable message.
+    """
+    current = _current_duplicates()
+    baseline = _load_baseline()
+
+    new_or_changed = []
+    for key, modules in sorted(current.items()):
+        if key not in baseline:
+            new_or_changed.append(
+                f"  + {key}  (NEW duplicate; registered in: {modules})"
+            )
+            continue
+        if modules != baseline[key]:
+            new_or_changed.append(
+                f"  ~ {key}  "
+                f"(modules changed: was {baseline[key]}, now {modules})"
+            )
+
+    if new_or_changed:
+        pytest.fail(
+            "Issue #239 CI guard: detected duplicate route registrations "
+            "that are NOT in the tolerated baseline.\n"
+            "\n"
+            "If you added a new @app.get/post/... in main.py for a path "
+            "that a router module already serves, please move the handler "
+            "into the router and delete the main.py copy — the router "
+            "version wins on request routing anyway, so the main.py copy "
+            "is dead code that just creates drift risk.\n"
+            "\n"
+            "Offending entries:\n"
+            + "\n".join(new_or_changed)
+            + "\n\n"
+            "Baseline lives at "
+            f"{BASELINE_PATH.relative_to(BASELINE_PATH.parent.parent.parent)}."
+        )
+
+
+def test_baseline_only_lists_real_duplicates():
+    """Catch baseline drift in the other direction: if an entry in the
+    baseline is no longer actually a duplicate (because someone deduped
+    it manually), the baseline is stale and should be shrunk so future
+    re-introductions of that duplicate get caught.
+
+    This test is informational — it does NOT fail the build today (the
+    audit's main concern is *new* duplicates, not stale baseline
+    entries). It prints a warning so the next baseline regeneration
+    can clean things up.
+    """
+    current = _current_duplicates()
+    baseline = _load_baseline()
+    stale = sorted(k for k in baseline if k not in current)
+    if stale:
+        # Use warnings instead of fail so this is friendly housekeeping,
+        # not a CI blocker. The other test catches the actual safety
+        # concern.
+        import warnings
+
+        warnings.warn(
+            f"duplicate_routes_baseline.json contains {len(stale)} entry/entries "
+            "no longer present in app.routes — consider regenerating the baseline. "
+            f"Stale: {stale[:5]}{'...' if len(stale) > 5 else ''}",
+            stacklevel=2,
+        )
+
+
+def test_router_handler_is_the_one_that_serves():
+    """Pin the empirical claim from PR #286: for every duplicated
+    (method, path), the FIRST-registered handler is in a router
+    module, not in main.py. If this ever flips — e.g. someone moves
+    include_router calls to the bottom of main.py — duplicate routes
+    start silently changing which handler runs. This catches that
+    rearrangement immediately.
+    """
+    import main
+
+    first_seen: dict[str, str] = {}
+    for route in main.app.routes:
+        path = getattr(route, "path", None)
+        methods = getattr(route, "methods", None)
+        endpoint = getattr(route, "endpoint", None)
+        if not path or not methods or endpoint is None:
+            continue
+        for method in methods:
+            if method in ("HEAD", "OPTIONS"):
+                continue
+            key = f"{method} {path}"
+            if key not in first_seen:
+                first_seen[key] = endpoint.__module__
+
+    main_winning = sorted(
+        k for k, mod in first_seen.items() if mod == "main"
+    )
+    # The duplicates we tolerate are router-first. If main is the first
+    # registered for any duplicated path, the router copy gets shadowed
+    # instead, which would invalidate every assumption made in audit
+    # rounds 5 and 6 about "the router version is canonical."
+    baseline = _load_baseline()
+    main_first_in_baseline = [k for k in main_winning if k in baseline]
+    if main_first_in_baseline:
+        pytest.fail(
+            "Issue #239 invariant broken: for at least one duplicated "
+            "(method, path), main.py is now registered FIRST and is "
+            "serving requests instead of the router copy. Audit rounds "
+            "5 and 6 assumed the router handler wins.\n"
+            "\n"
+            "Affected entries:\n"
+            + "\n".join(f"  {k}" for k in main_first_in_baseline)
+            + "\n\n"
+            "Most likely cause: someone moved app.include_router(...) "
+            "calls in main.py to after the @app.get decorators. Move "
+            "them back to before the @app routes (currently around "
+            "line 3316)."
+        )