Towncrier adds double heading

[jaraco]

The change that introduced this error was 529419e, which was introduced by running tox -e finalize. It seems towncrier is no longer able to properly finalize the release, so while #2665 addresses the symptom, the root cause, probably introduced in #2654, will still need to be addressed.

Originally posted by @jaraco in #2665 (comment)

@webknjaz, can you triage and propose a solution?

[webknjaz]

Interesting... @jaraco Can it be that in your pre-cached .tox/finalize/ folder (on your laptop, in the repo checkout) towncrier is v17? It would explain what happened, probably.

[webknjaz]

But it's still a bit of a mystery to me why the first version doesn't include the underline.

[webknjaz]

Alright... This is reproducible with the latest towncrier.

Also, I noticed that running towncrier vs towncrier --draft produces different results — the latter returns a proper title while the former writes a broken one into the file.

[webknjaz]

So apparently this happened in pip almost two weeks ago: pypa/pip#9828 (comment). I'll try to debug this further later.

[webknjaz]

@jaraco feel free to assign the issue to me (I don't have the power to self-assign here)

[webknjaz]

There is a Towncrier regression that makes normal and draft runs produce different outputs: twisted/towncrier#105. A possible upstream fix is present but looks a bit stale: twisted/towncrier#303.

[vytas7]

I've been struggling with a similar or the same issue: twisted/towncrier#345 (see also: https://github.com/twisted/towncrier/pull/299/files#r619798640).