How we know they’re different people:
Greg Allen Harris was known not only for being both a mediocre starter and a mediocre reliever, he also wore a funny-looking glove that could be worn on either hand, because he had the ability to throw with either hand. He even demonstrated this ability on 1991 Score.
Gregory Wade Harris, meanwhile, posted some pretty decent numbers as a reliever before being converted into a mediocre starter himself. He threw with his right hand only.
How we’re not sure they’re different people because we’ve never seen them in the same room together:
They both pitched, and for the most part they pitched in opposite leagues. Back before interleague play and the Internet, who really knew what was going on in the other league? There’s almost no way to tell that these alleged Greg Harrises were not just one person going back and forth from league to league, being average.