Awesome idea, @PabloC! 
Ironically enough, I can tell you I just did not create 2 topics thanks to it. One was a duplicate from this, the other one about /email_preferences. 
What do you mean? How about just checking if user clicked on any “similar links” and then canceled the new topic, at any point in time? It shouldn’t be too hard to at least think of ways! 
I’d be interested in making it happen even into the interface if you tell me this is something you might accept a PR on. 
But I do have an awful history of wanting to do PR’s (to discourse) and never following through.
Just like with stackoverflow, I often use this instead of searching, as it works so much better.
Why not having a link to “more” and then a page full of similar results?
As for background work, if users and mods could mark topics as duplicated (or at least related) it would make awesome data for this feature to find similar topics.