Keys getting stuck

if using usb port, try different port. make sure there is no mapping on your mouse to enter anything either. you can program mouse buttons/actions to do stuff and you might have an "s" in clipboard and you are executing the paste "button"

Sounds like a mechanical problem with the keyboard - i.e., the 'S' key is literally sticking down and then eventually releasing. I once had a problem when a grain of instant coffee lodged between two keys and stuck them together, so that a press on either typed both characters...

Over time, keyboards get very dirty inside as crap falls between the keys. If the problem is with one key specifically, you can probably solve this by popping the key top off gently with something like a car key and cleaning out any gunk underneath (the key top will fit back on with a firm press). Make sure that key rises cleanly when you release it.

You may need to clean the whole keyboard. A start would be to hold it upside down and shake it gently - you may be surprised by what falls out. Better to do this with the keyboard disconnected, of course.

In the final analysis, keyboards do fail eventually and aren't expensive to replace.

Just try another keyboard to see if the problem comes from your keyboard or not. This should be the first thing to try before deciding what to try next.

Have you tried playing with the value for Repeat Delay in the Keyboard > Speed settings. Set it to longest and see what happens. Change it notch by notch back until you find that strange bug occurring again, then obviously go down one.

My laptop has an issue where sometimes the repeat delay becomes too long, moving the slider back and forth then saving the changes resets this bug.