AFAIK, Lovecraft himself said that he had made it up, and no occultist, mystic or witchhunter before Lovecraft has ever mentioned it in my admittedly limited knowledge. (I'm more Scully than Mulder -- I have no intention to believe unless it's lying in front of me in a brightly lit room and it's dead ;-).

The two or three books I read that claimed to be contain parts of the Necronomicon were cheaply produced by small presses preying on wannabe Goths, small-time Satanists, D&D geeks and people looking for something to mock. I checked them out when preparing to GM a Call of Cthuhlu game and found that they put me in a mood not entirely suitable for a horror game. I also didn't notice any of my sanity slipping. (Nor did anyone else, which is the more reliable indicator -- "as crazy as ever", my roommates said.)

Of course, it can be impossible to prove that a certain thing doesn't exist.