The product code you’ve entered has already been activated by a different Steam account
sweet,
can i get another code?
Same issue here, received a code marked as a duplicate. Having a blast listening to my friends play the flight though.
Same here, I can’t activate my key.
I have the same issue here. I haven’t heard any communication from the team yet if they are fixing the issue or not.
Same here, still haven’t seen 343 acknowledge the issue anywhere 
They are working on resolving it and have been since last night Source: Brian Jarrard Community Director at 343 Industries https://twitter.com/ske7ch/status/1441433124081070082
343 are aware of issues with duplicate/invalid Steam keys and will be addressing this.
Please see my post here and follow that thread for further updates:
https://www.halowaypoint.com/en-us/forums/243780d3b80b4bb6ac7917a8942286db/topics/psa-halo-technical-preview-2-updates/511173a1-85aa-4941-b02f-12645a4917ea/posts?page=1#post6
<mark>For anyone dealing with the duplicate/invalid Steam code issue - please file a support ticket on the Insider Support Site:</mark>
https://insidersupport.halowaypoint.com/hc/en-us