Yes, the same flight number can be used to refer to multiple flights active at the same time. This happens fairly regularly if a flight is heavily delayed. The previous days flight may still be active the same time as the current days flight takes off.

Yes, a flight number can be used twice in a single day, and this is actually a standard practice in the aviation industry. Airlines often assign a single flight number to a "multi-leg" route. For example, Flight AA100 might fly from Los Angeles to New York, and then after a short layover, the same flight number is used for the continuation from New York to London. Even though it is the same number, it is technically two different flight segments. Occasionally, if a flight is severely delayed from the previous day, you might even see two flights with the same number in the air at once—the "delayed" flight from yesterday and the "on-time" flight from today—though the airline will usually append a letter to the delayed flight's number (e.g., AA100A) to avoid confusion for Air Traffic Control. In 2026, tracking apps like FlightAware or FlightRadar24 are sophisticated enough to distinguish between these segments, but you should always verify the specific "Departure City" to ensure you are looking at the correct leg of the journey.