Are there any chances that the Fischer who reappeared in 1992 was not the real one? As in title above, are there any chances that the Fischer who reappeared in 1992 (and then later on), was not the real Bobby? If this is not the case, what do you consider as "proof" that he was indeed the real Fischer?
At the very least we know that in 1992 a man that looked and sounded like an aged version of Bobby Fischer played Boris Spassky in a match, and won it. Assuming that this old man was actually not Fischer, we must think of plausible alternatives for who it possibly could've been. What old man besides Bobby Fischer will look like Bobby Fischer and be able to play well enough to beat a former world champion? How couldn't this mysterious veteran be at least somewhat famous in the chess world (being a strong player that is also a Fischer look-alike)? And how did he manage to dupe the chess world (including all the people who knew him on a personal level) into thinking that he was Bobby Fischer?

It is very difficult to give plausible answers to the above questions, and since there is no real indication that it wasn't the real Bobby Fischer to played Spassky I will go ahead and use Occham's razor to conclude that the most reasonable explanation is that it was the real Fischer that played in the 1992 match.