I just tried putting my steam key in and am being told that this is a duplicate product code.
Same here. 
I had the same issue and just submitted a support ticket.
yall are using the mcc code bruh
same here too
Me as well. Definitely not using the MCC code I copied the code from the Halo Infinite flight message specifically.
this is also happening to me
Also happening to me
Thank gosh I was not the only one! Im assuming it was just the same key we had for the last flight lol
same here, sucks hopefully it gets sorted for us.
Same here tried using it and came up

> 2533274796670669;3:
> I had the same issue and just submitted a support ticket.
Where did you submit the ticket on?
Same issue for me…definitely the right code i am typing in.
im getting the same thing
the same unlucky thing :C
> 2535409103705542;12:
> > 2533274796670669;3:
> > I had the same issue and just submitted a support ticket.
>
> Where did you submit the ticket on?
I posted the issue on another forum post and got a invite to the infinite support page. https://insidersupport.halowaypoint.com/hc/en-us . But click infinite and submit a ticket. The website is being a bit buggy unfortunately, showed the support ticket was gone and now it’s back.
Yeah I am having the same problem. 100% using the Halo Infinite Multiplayer Technical Preview code.
The automatic bot responding to Duplicate Key tickets comes back with Halo Insider Support can not re-issue/ make new Steam keys. So we might be out of luck
I’m having this same issue as well.
Same issue, time to wait.