diff --git a/browse/test/sidebar-ux.test.ts b/browse/test/sidebar-ux.test.ts
index c03b18cd..a96f57a2 100644
--- a/browse/test/sidebar-ux.test.ts
+++ b/browse/test/sidebar-ux.test.ts
@@ -940,6 +940,40 @@ describe('chat toolbar buttons disabled state', () => {
   });
 });
 
+// ─── Chat message dedup ─────────────────────────────────────────
+
+describe('chat message dedup (prevents repeat rendering)', () => {
+  const js = fs.readFileSync(path.join(ROOT, '..', 'extension', 'sidepanel.js'), 'utf-8');
+
+  test('renderedEntryIds Set exists for dedup tracking', () => {
+    expect(js).toContain('const renderedEntryIds = new Set()');
+  });
+
+  test('addChatEntry checks entry.id against renderedEntryIds', () => {
+    const addFn = js.slice(
+      js.indexOf('function addChatEntry(entry)'),
+      js.indexOf('\n  // User messages', js.indexOf('function addChatEntry(entry)')),
+    );
+    expect(addFn).toContain('renderedEntryIds.has(entry.id)');
+    expect(addFn).toContain('renderedEntryIds.add(entry.id)');
+    // Should return early (skip) if already rendered
+    expect(addFn).toContain('return');
+  });
+
+  test('addChatEntry skips dedup for entries without id (local notifications)', () => {
+    const addFn = js.slice(
+      js.indexOf('function addChatEntry(entry)'),
+      js.indexOf('\n  // User messages', js.indexOf('function addChatEntry(entry)')),
+    );
+    // Should only check dedup when entry.id is defined
+    expect(addFn).toContain('entry.id !== undefined');
+  });
+
+  test('clear chat resets renderedEntryIds', () => {
+    expect(js).toContain('renderedEntryIds.clear()');
+  });
+});
+
 // ─── LLM-based cleanup architecture ─────────────────────────────
 
 describe('LLM-based cleanup (smart agent cleanup)', () => {
diff --git a/extension/sidepanel.js b/extension/sidepanel.js
index bfc28e88..9e6626fc 100644
--- a/extension/sidepanel.js
+++ b/extension/sidepanel.js
@@ -102,7 +102,17 @@ let agentContainer = null; // The container for the current agent response
 let agentTextEl = null;    // The text accumulator element
 let agentText = '';        // Accumulated text
 
+// Dedup: track which entry IDs have already been rendered to prevent
+// repeat rendering on reconnect or tab switch (server replays from disk)
+const renderedEntryIds = new Set();
+
 function addChatEntry(entry) {
+  // Dedup by entry ID — prevent repeat rendering on reconnect/replay
+  if (entry.id !== undefined) {
+    if (renderedEntryIds.has(entry.id)) return;
+    renderedEntryIds.add(entry.id);
+  }
+
   // Remove welcome message on first real message
   const welcome = chatMessages.querySelector('.chat-welcome');
   if (welcome) welcome.remove();
@@ -577,6 +587,7 @@ document.getElementById('clear-chat').addEventListener('click', async () => {
   } catch {}
   // Reset local state
   chatLineCount = 0;
+  renderedEntryIds.clear();
   agentContainer = null;
   agentTextEl = null;
   agentText = '';