I do have some doubles for some games. If I find a sealed game I'll buy another used one to actually play, or if one is in extremelly good condition I might just get another copy to play without having to worry about messing up my good one. Or I might have a few copies of one title if they're variants. Usually I don't plan on having multiple copies though.

I would pick up any good game I come across if the price is right, I do tend to trade them or sell them. Often I pick up good games to bundle with commons, it tends to be the only way to really get rid of common games. I end up buying bundles or small collections of games to get the titles I want, I end up with extra commons as a result.