A minor meme of /b/ that was first posted 2006/03/05, featuring a character who was drawn on 2005/05/14, but with the face erased. It came with the message "I've given you a blank check, go crazy". Within an hour, over 100 different faces were given to Doppelgänger.
Doppelgänger is literally a window to the past in which you can look at mems of old.
The name Doppelgänger most likely came from the Sandwhich guy post with the text "Nice try Doppelganger! Save it for Queen Doppelpoppolus!"