Activating the game on different operating systems.

I reinstalled the system, reinstalled Heroes cleanly from the disc, downloaded all the patches through the Ubisoft launcher, and entered my Uplay account details. Then, it asked for a key. When I entered the key, I received the message "key already used." The logic seems to be there – new Windows, new computer – but is it normal that the key isn't linked to the account? And what should I do now? Buy a new license?

I found a similar question on the internet, but it remained unanswered. I'm counting on you, forum members! What should I do?