Click to see Spoiler:
Given that on the 50th anniversary they're brining back the Zygons - a species that can morph into another figure, the 10th and Rose are going to indeed be two Zygons in disguise in the end and try to kill the 11th and Clara.

Of course we may be seeing the 10th and Rose from another time - say around the time of "Love and Monsters" but I just have this feeling this is what is going to be done, and it'll irritate several of the 10th fans, never mind that we've had doppelgangers of previous Doctors as bad versions of themselves or monsters that cloned his looks:

Second - "Enemy of the World" (Sylvester or some sort of megalomaniac)
Fourth - "Meglos" (Meglos), "Android Invasion" (An android that looked like him)
Fifth - "Arc of Infinity" (Omega)
Sixth - well here most noticeably in the Fifth Doctor Story "Arc of Infinity" as a different Timelord.