Well a lot of these are obscure games or TV references (mine's not an obscure game but old and minute enough to not be duplicated, old avatar is popular but again, minute). There's so many pictures you could do that duplicates are rare. If my old stomping grounds had Avatars we'd have half the people with Sephiroth or Kefka Avatars.
That said, one close thing to duplicates I'm getting sick of would be that weird picture of a guy with a toothy grin, wide eyes, and eagerly clenched fingers. I first saw it making a Squirtle, but has been used to create a number of similar-looking ones.