I sometimes have to check exactly which tags I used for similar/related articles so they match. There could be 10 stories related to "red cars" but if someone puts just "red car" then the story is alone.
Perhaps a button near the tag entry that the user can click to find similar tags already in use.
That is the basic concept and I'm sure a lot more can be done with it. Maybe even an automatic parsing that the user doesn't know about.