Doppelganger Race! is a recurring Challenge Act featured in the console/PC version of Sonic Generations that is unlocked after clearing every Stage in their respective eras. It is marked on the Challenge Act Gate with a gray version of the Extra Life symbol.
In this Challenge Act, Modern Sonic or Classic Sonic must race against a faint version of himself, the "doppelganger", in an exact copy of the specified level. The doppelganger seems to be in his own version of the level, as when the player destroys an enemy before the doppelganger, the doppelganger jumps on an invisible enemy in that exact location, no matter how much time has passed. There are no advantages given to either Sonic or the doppelganger. In order to complete the mission, the player must simply reach the Goal before the doppelganger does.
- This is the only Challenge Act to be playable for every level in the game, Classic and Modern.
- This is the only Challenge Act to be an exact copy of the stage it is based upon.
- This mode bears a long resemblance to the ghost race in Sonic R.