John Carpenter's Tales of Science Fiction: Interference Pattern (John Carpenter's Tales of Science Fiction: Interference Pattern #2)

David makes it back to our Earth in one piece, but wants to halt any further testing until they understand what went wrong. However, Dr. Nestor is determined to forge ahead, and begins construction of a new FTL vehicle. Then one night an unexpected visitor shows up at Nestor's lab. It's David, but not our David. This Alt-David somehow also ended up here after the test. In his universe, he lost his family in a car wreck. He's been secretly watching our David and wants to replace him, getting his life and family back, and he's ready to do whatever it takes to make that happen.