Yes! I played a doppleganger once!
However, it was back in 2e, and it wasn't really my character. It just happened once: in the campaign we played at that time, I eventually grew bored of my character (a fun loving halfling thief). As such, the DM proposed me to play a Doppelganger assassin. So, my PC halfling once decided to leave the group, and a Doppelganger assassin who had followed our group for some time took his place. As such, the players still believed I was the halfling.
The funny thing is, that I managed to play the Doppelganger as he couldn't know everything the halfling kenw (of course), saying some strange things, not recognizing some NPCs, etc. Hey, the other players never figured out anything! I had even my new character sheet openly on the table. Besides me was the player of the mage. This (evil) mage was paranoid and had cast detect spells of all sorts on all the PCs... except me!! He later told me that he couldn't possibly see a threat in a halfling.
Hence, as a Doppelganger I did wait three or four adventure, and then, there was a dificult fight. I saw it as my "chance", and attacked the players. I killed two. I failed against the third who was the evil mage, because he had stoneskin then teleported away. I will remember for a long time, how he laughed maniacally for at least half an hour...