Doubling up is one of the most common medication mistakes and it can be harmful. Taking two doses too close together can push the amount of drug in your body above the safe range, leading to stronger side effects or, with some medicines, dangerous overdose. This is especially true for medicines that affect the heart, blood pressure, blood sugar, blood clotting or the nervous system.
The safer approach is usually to take the missed dose only if there is still a reasonable gap before your next one, and otherwise to skip it and resume your normal schedule. Never try to catch up by combining doses without specific advice that says to do so.
A few medicines do have approved catch-up instructions, but these come from the patient leaflet or a healthcare professional, not from guessing. If you are ever unsure whether you have taken a dose, it is generally safer to skip than to risk taking it twice, but confirm with your pharmacist.