I just stumbled across this thread and thought I'd share some hints against things that have happened with me a few times. Maybe one of these will help; maybe not.
Letters do repeat if you hold down the keys too long as you type. I suppose we have a way to turn that off or change the length of time before it starts repeating in the Settings somewhere, but it's not an issue that hits me very often, so I haven't turned it off.
Sometimes as I type, letters double up anyway, even if I don't hold down the keys very long. I hope that's not a sign that my keyboard will die, since it's integrated into my laptop and I would have to replace the whole computer if it happens very often.
Make sure your Caps Lock key (CapsLk on my computer) is off. Since you probably type your user name before you enter your password and can already see how your typing shows on the screen, this probably isn't your issue.
If you learned typing the way I did, you rest your fingers on ASDF and JKL; (the home row keys) while you move to form the words and such. Also, if you learned the way I did, you learned on manual typewriters that required quite a punch to activate a key and make an impression of a character against the ink ribbon and onto the paper. Computer keyboards are so much easier to touch and activate characters that sometimes, especially if you are tired or you don't hold your arms up as high as you used to, you might inadvertently add characters to your intended typing.
If you learned typing the way I did, the manual typewriter had no key for the number 1 and required you to use lower case L as a 1. It even let you use capital O instead of zero if you wanted to do that. I had to unlearn this early on in my career with computers. I had to retrain myself to use my left pinky for a 1 and my right pinky for a 0 every time.