Sometimes there will be text after my signature, such as "please ping me in replies since I don't watch this page" or perhaps my comment was in <small></small>. In those cases, the reply tool ignores the fact that I already have a signature in my message and sticks one on the end anyway.
Topic on Talk:Talk pages project/Replying/Flow
There was a similar request on Phabricator: T268558
Aha, thanks very much! Subscribed to that ticket.