If Alice answers "reverse the polarity" in January, and then in July Bob answers with "try reversing the polarity," should we flag the second for moderation action and deletion? It seems like such an answer is just clutter, which SE is generally against. From a quick search on the master Meta site, it's implied that such answers should be deleted.
Of course, if the later answer is substantially better (more clearly written, or better supported by references, perhaps) it's worth keeping. It's harder to say when the answers are posted a few minutes apart, as Bob may well have been making a good-faith effort to avoid duplicates. Also, if Bob's answer is slightly better, reasonable people may disagree about whether it's worth keeping or just clutter.