packager: WARN on duplicate commit messages in release notes

[davelab6]

https://github.com/googlefonts/roboto-flex/releases/tag/3.100 used to say

* Update for 3.00x prerelease
* Parametric update
* VF Update
* VF Update
* VF Update

because the commit title messages were written in a vague way, but I am grateful that the commit's body messages did actually say more about what happened in this release, so I could update the release note manually to

* Update for 3.00x prerelease, fixes #298 #271 #303
* Parametric update to parametric widths
* Cyrillic updates by Ilya Ruderman
* Fix dots #313 and monetary notes #312
* Fixed YTAS f, no crash wght1000 over opsz axis

While I've asked for more care to make meaningful commit titles, so that when the release script is run, it can generate useful release notes, I'd like to ask for a small simple feature in the packager itself that will WARN when there are repeated commits in a release and ask the user to supply a manually corrected release note text.