Comments: Sometimes, on older keyboards and/or ones that have seen a lot of use, keys may stick when you press them down (and refuse to pop back up), or they may cause bouncing or repeating (more than one character showing up after pressing a key only once). Sometimes a key may not generate a keystroke the first time it is pressed and may have to be pressed down hard in order to recognise.
Cause: These problems are usually caused by dirt and debris within the keyboard, and can usually be fixed. On very old keyboards, these symptoms can just generally be a sign of wear-out of the internal mechanisms.
Solution #1: If the problem is that of keys repeating, and this happens on many different keys on the keyboard, then this may not be a physical problem but rather the “typematic” setting. This is a feature that causes a keystroke to repeat automatically when you hold the key down. Check in your BIOS and in the Keyboard section of the Control Panel for a setting to control the typematic delay, which is the amount of time a key must be held down before it starts repeating, and increase it if you are finding the key repeating too quickly.
Solution #2: Clean the keyboard. If you are only having a problem with one key, you may want to try first just cleaning around that key cap. Of course once you open up the keyboard (if you do) you might as well clean the entire inside of the keyboard. Be careful about what you open; some keyboards cannot be reassembled once opened!