Probably introduced in rP3dd92a78ac2a (other commits since our last update don't seem to touch this code).
When someone pushes their commit, the revision gets closed, which gets mentioned in the task:
xxx added a commit: rXXabcdefabcdef: Use xx for xx to fix xx
However, it now also generates a mention:
xxx mentioned this in: rXXabcdefabcdef: Use xx for xx to fix xx
It seems a bit double. It doesn't bother me, but some coworkers were somewhat confused and thought they did something wrong.
(The commit(s) was not changed in any way after running arc amend)