If you see this message, it means you already have access to this product on your account. You cannot activate the same product twice on Cambridge One. 

This may happen if:

• You already used a different code for this product
• You are trying to add a second code from the same product before the first one has expired.

Once your code expires, you can activate a new code for the same level.
If you're not sure when your code expires, you can check the expiry date on your account: