Yeah its his real twin.
Basically they were evacuating a base on a planet with extreme static interfearance or something. With a new storm on the way, it created a 8 year window which they could teleport in and out. Two beams were used to get get a lock on Riker (who was the last one out), one went to his ship, the other was bounced off the stratosphere and materialized back on the planet.
So basically he was trapped on the planet for 8 years, still loving Diana, still hating his father.
This scenario re-surfaces in a Deep Space 9 episode where his twin tries to make something out of himself as a member of the Machki(sp?) (Cardassian Resistance).