Expanding on eykanal's suggestion of leaving a comment, I sometimes leave the following comment:
It's not clear what this answer adds over previous answers that already address these points.
and, if the author of the answer is new or not a regular contributor, I might also include in the comment:
Answers on Academia.SE are expected to offer a fresh take, rather than just reiterate existing answers; see What are we generally looking for in answers.
I invite anyone who witnesses dogpiling to "steal" this formulation (or some variation of it) and leave this comment yourself :)
Also note that comments that just repeat things that have already been said should be flagged. The appropriate flag depends on the situation, but I find that "too chatty" is often suitable.