First, split the symptom in two
Keyboard problems come in two flavors: keys that sometimes don't register at all (missed input), and keys that type twice from a single press (chatter, as in 'thhe' or 'doouble'). The causes and fixes differ, so identify which one you have before doing anything else.
Open our keyboard test and press the problem key about twenty times at your normal typing speed. If the virtual keyboard sometimes fails to light up, that's missed input; if the recent-keys history shows the same key twice in a row from one press, that's chatter. Also note whether it's one key or the whole board misbehaving — if it's everything, the culprit is usually the connection (cable, wireless receiver, USB port) or software, not the keyboard itself.
Missed input: usually debris or a worn contact
If a single key intermittently fails, the odds-on cause is debris under the keycap. Pull the cap (don't force it on membrane boards) and clean around the switch with compressed air or a soft brush. Crumbs and dust physically blocking switch travel is genuinely the most common finding.
If cleaning doesn't fix a mechanical keyboard, the switch contact itself is failing. On a hot-swap board, pull the switch and press in a new one — done, for a few dollars including the tool. On a soldered board, replace and re-solder just that switch or use a repair shop. For a laptop's built-in keyboard, a single dead key often means a damaged scissor mechanism or rubber dome underneath; parts are hard to source, so an external keyboard is frequently the practical answer.
Typing twice (chatter): the switch is wearing out
When a mechanical switch's contacts degrade, one press produces several electrical bounces, and if the firmware's debounce can't filter them, you get a doubled character. It's exactly the same failure mode as mouse double-click malfunction. It progresses faster in humid environments, and typically starts on one key before spreading to others.
Try software mitigation first. On QMK/VIA-compatible keyboards you can raise the debounce time (default 5 ms → 15–20 ms), and some vendor software offers the same setting. Chatter-filter utilities that enforce a minimum interval between repeats also exist, though they can interfere with games that need rapid key taps. The lasting fix is the same as for missed input — swap the switch on a hot-swap board, solder a replacement otherwise — and within warranty, chatter is a valid defect claim.
When the whole keyboard acts up
If many keys misbehave at once, suspect the connection, power, or software before any individual switch. Work through: a different USB port (directly on the PC, no hub) → a different cable (if detachable) → for wireless, fresh batteries and receiver placement (metal desks and USB 3.0 ports cause interference) → finally, plug into another PC. If it works fine there, your keyboard is healthy and the original PC's drivers or software are the problem.
If keys only drop while gaming with several held down, that may not be a fault at all — it may be your keyboard's rollover limit. Check with the simultaneous-input (NKRO) counter in our keyboard test: 2–3 keys is normal for office membrane boards, 6 or more for gaming keyboards. That's a specification, not a defect.