There have been rumors of robinhood simply being buggy for some time before this. Things like orders being processed after being cancelled, orders being processed out of order causing conflicts, orders being executed massively delayed after the client threw an error message. Basically, if their database infrastructure is buggy and unstable, then combined with heavy load, something like this could happen. And notably robinhood does not provide use client side action logs, only server side actions logs on demand.
I think it could have happened, but I very much doubt they ever did it intentionally, and it is practically guaranteed people would claim this happened to them, regardless of if it did actually happen to them.
If they process an order 'late' the price change could swing in their favor and they would end up netting the difference. FXCM got caught for this in FX years ago, skimming fractions of pennies off the trades for millions in profit. I'd be interested to know if those late trades always went in one direction.
I think it could have happened, but I very much doubt they ever did it intentionally, and it is practically guaranteed people would claim this happened to them, regardless of if it did actually happen to them.