Hello! Today I saw that I received a Halo 2 flight product key and I tried today to use it and it’s already a duplicate and I cannot use it anymore. Please someone help me! 
The flight ended weeks ago and with it all keys were revoked which is why it’s saying yours is a duplicate. You missed out on that flight I’m afraid but the game’s out now and there’ll be more flights down the road for Halo 3, ODST, and 4.