To double-check is to look at something again so you can be completely sure it is right, safe, or still there. You already checked once; now you check the same thing a second time — the lock, the spelling, the numbers, whether the passport is in the bag. It does not mean you check two different things, and it does not mean you are bad at checking. Careful people double-check. You can double-check a fact, a door, a list, or someone else’s work (kindly, not as a scolding). The word is friendly and everyday: home, school, and offices all use it. In Taiwan workplaces you will also hear the English “double check” dropped into a Mandarin sentence. “Check twice” is almost the same idea, but double-check is the set phrase people reach for.