OK, say, we have a popular question with a lot of comments and answers. It can be a lot of links there and the bigger is their amount, the more probably you can miss such link.
While inserting links in my own answer, it would be nice if I'll be warned somehow that this link is already mentioned in other answers. Technically it is not difficult and most probably better to be solved on client side. As for UI - well, there is actually a room for discussion how it's better to be implemented.
But I'm asking only about concept, so my question is humble) Should we bother about repeating links?