checkChangelog takes the base branch's CHANGELOG: a heading the base already
carries N times may appear N times on the PR (inherited released history), and
only a further copy is a duplicate. gstack-restamp passes origin/<base>'s
CHANGELOG at the plan's base sha, so a base like gbrain's master, which heads
one version twice, can be stamped; a duplicate the PR adds is still refused
with nothing written.
Co-authored-by: capy-ai[bot] <230910855+capy-ai[bot]@users.noreply.github.com>