I just got my invite to halo insider hour ago and when I try to activate my code it says duplicate code
Can you help please
Same thing happened to me, dont know how to fix it
Same for me. Had problems with the page loading and when it finally did It was apparently a duplicate code 
This is a known issue with Infinite Steam codes. 343 have identified the source of the invalid Steam keys. They’re working on getting new ones sent out to people who received invalid ones. To set expectations, it likely won’t happen tonight but the team believes they can address it tomorrow
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