Halo Infinite

I enter my code on steam and I get “Duplicate Product Code”

My brother is playing on Xbox. He gave me his PC code to try. It says the same thing…
How do I get a valid code?
Did someone generate codes and stole them?

I am having this issue for the current flight. I have a Steam product code I get from halowaypoint.com for the Halo Infinite Test Flight and I get the message “Duplicate Product Code” and I haven’t redeemed anything for Halo on Steam before. So I don’t know what to do exactly.