Future Past (1989)

Harlan, a daydreaming video store clerk with religious fanatic parents, needs more computing power for an experiment he's doing in his room. To get this power, he taps into a company supercomputer run by a slightly deranged professor doing some strange experiments of his own. Something in the two experiments gets crossed, and a future version of Harlan is brought into the present. The future-Harlan is greedy, cynical and wants to use his knowledge of the future to gain wealth and power. It's up to the present-Harlan to stop him, but how do you outwit someone who's already done what you're going to do?