Steps to reproduce:

1. On a talk page, click "Reply" on an existing comment.
2. Go into advanced mode.
3. Click on the edit summary field to place your cursor in it.
4. Press Ctrl+Enter to save your reply, with the edit summary field still selected.

What happens?:
The reply is duplicated. An edit conflict warning briefly flashes before the first copy appears.

What should have happened instead?:
The reply should only be saved once.

Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc: Firefox 91.0 x64 on Windows 10. Example at w:User talk:Rummskartoffel/sandbox.