Uplay PC CD keys are unique and can only be used once. Once they are used, they are bound to the Uplay account that was logged in at the time of initial activation.
You do not need to create a new Uplay account. If you are receiving this error, chances are you already have more than one Uplay account. You can simply log in to your other known Uplay account(s) to find which one has the game registered.
Steam: Games purchased through Steam will appear in the "My Games" tab in the full Uplay PC application. If you are trying to play a game originally purchased through Steam, you can either launch the game through the Steam application or through Uplay itself.