I can't guess if you have some keyboard driver or enhancer installed or even if such is required. All I have is HP and Windows 10. That's not enough detail but there are priors at https://www.google.com/#q=can%27t+type+double+letters which appear to happen over and over across Windows versions.
Most common answer is that Filter Keys is on.
After the latest Windows 10 update, I am having difficulty typing, especially words with double letters; e.g., letter, outlook. backspace key not working and other typing issues as well. HP support of no help. Can you provide assistance or suggestions? Would be most grateful.