packager: WARN on duplicate commit messages in release notes

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.