How do you convince Cisco Ramon that youre the real Barry Allen? Why, you reference Star Trek II: The Wrath of Khan, obviously.
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With Cisco confronted by two Barrys, the past Barry claims that hes the real Barry due to all of the times that hes watched Wrath of Khan with Cisco. As for future Barry, he then goes on to describe exactly what Cisco does each time the film ends.
The end result was an über-confused Cisco Ramon who was having a Wrath of Khan overdose.