I replied to a post which had a block indent template and it duplicated that template. See dif
Topic on Talk:Talk pages project/Replying/Flow
It sounds like a fix is on the way, but I'm not sure exactly when it will reach the wikis.
Thanks for the report. Yeah, I'm pretty sure that the fix for T289873, which I'm already working on, should fix this problem as well. (They don't obviously seem alike, but it's actually a similar structure on the page, where a comment is partially generated by a transclusion.) I'll post some details on the task and ensure that we test this case afterwards.