I have decided that my favored explanation is that they've been working on a "is it possible to clone humans?" episode. The answer turns out to have been yes, but the clone immediately took off to create his own life, Tom Riker style. This is based on the fact that the ONLY reasons I'm not confidently saying "I saw Adam Savage on the plane" are: (a) he was wearing the wrong style of clothing and (b) his glasses had the wrong style of frames.

Alternately, of course, he has an identical twin...