Maybe everybody agrees that answers that add nothing should be deleted. The problem however is with the "add nothing" bit. People differ about where the bar lies between something and nothing.
On the one hand there is this feature request that comes from a moderator on SO. The feature would be (in brief) to add a way to vote to delete an answer as a duplicate of an earlier one on the same question.
On the other hand, when I've flagged answers as duplicates of earlier ones on SO, I've rarely been able to get the desired results. Maybe I did not make enough of a case for it, or the moderator who handled the flag placed the bar between something and nothing somewhere else than I do, which is likely. The closest the duplicate answer is to be a cut-and-paste of an earlier answer with minor modifications (yes, some folks do this), the greater the chances of success. And there has to be a significant time difference between the time the answers were posted. Simple problems don't have very many significantly different solutions, combined with the Fastest Gun in the West phenomenon you get a bunch of similar answers posted in a short time span. This cannot be helped.
In the meantime, you can downvote, which is unlikely to do much in the grand scheme of things (even with a comment), as the mass of users (on SO at least) tends to upvote anything which is not untrue, irrespective of whether or not it is adding anything to the earlier answers, pertains to the question, etc.