Solution: Called Apple. Set up a return. Christmas will have to wait until the end of January. I found out that it was a hardware issue that could be replicated in different degrees, depending on how the key was pressed. In-store 2016 MacBooks definitely do not have this issue. I've tested with every key on a few. Writing E would either appear twice or once, but with a delay (meaning lots of typos). Problem still persisted after resetting SMC, PRAM, and entering safe mode, so it's safe to say that it's a hardware issue.