This question already has an answer here:
Many questions on stack overflow are duplicates. These are often closed, but what would be nicer is if the question could be declared a synonym of another (by users of sufficiently high rep, or by votes).
Why do this? Well one reason duplicates come up is that there's lots of different ways of phrasing the same basic question. This means that when I type in the title for my question and it suggests related ones, it may completely miss the identical question due to it being worded completely differently. Having the ability to link different questions could go some way to alleviating this problem, and linking the good answers to the previous incarnation of the question to the newly asked question, instead of having "answers" consisting of stuff like:
P.S. This may well have been asked before, but I've checked all the suggested Related Questions and it doesn't look like it!