As the doctor is a time traveller he does indeed meet himself every so often (often by coincidence at anniversary and Christmas episodes). I believe the episode you're refering to is "The Day of the Doctor" which recently aired with Doctors 10 and 11 as well as the "War Doctor".

This however is not an uncommon premise. There have been quite a few episodes which contain more than one doctor (usually anniversary episodes). The most famous of these have been "[The Three Doctors][1]" and "[The Five Doctors][2]".

In addition there was an episode called "[The Next Doctor][3]" in which we were led to believe we'd met Doctor 11 (this was before Matt Smith had been announced).

Finally all 12 appear (some via TV trickery) in "[The Time of the Doctor"][4].

You can see a complete list of multi-doctor stories [here][5].


  [1]: http://tardis.wikia.com/wiki/The_Three_Doctors_%28TV_story%29
  [2]: http://tardis.wikia.com/wiki/The_Five_Doctors_%28TV_story%29
  [3]: http://tardis.wikia.com/wiki/The_Next_Doctor_%28TV_story%29
  [4]: http://tardis.wikia.com/wiki/The_Time_of_the_Doctor_%28TV_story%29
  [5]: http://tardis.wikia.com/wiki/List_of_multi-Doctor_stories