Well, I have imagined this to happen as follows:
I ask a question. A person answers it. In his post there is a link to an existing Stack Overflow post which deals with the same subject. I assume he must have gone through the post before and thought it was useful; hence the link.
So why can't we come out with a strategy to recognize such posts (i.e. those which are linked to in other posts)? I don't mean that we reward every such link. If the person who has asked a question marks an answer he has received, and that answer contains a link to another post on Stack Overflow, then the linked-to post should be rewarded somehow. Also, the person who has answered the question should be awarded a little extra rep, since he has referred to that post.