I received a code from Xbox though my Xbox One for a code for Halo 3: ODST campaign for the Halo: MCC. When I went to redeem the code, I was informed the code had already been used less than a day after I received the message. I contacted Xbox support to try to resolve the issue to see if maybe I could get another code so I could play it. After a brief and polite conversation with support, all was needed was a picture of the code given to me as proof and now they are going to contact the Halo team. Turns out the code I received had been redeemed beforehand. So hopefully my issue will be resolved soon and I will get to play some more Halo! Thank you for your support Xbox!
My reference number for the case is : 1295063222