User account licensing doesn't work when you're activating offline; there's no connection to the licensing server therefore you're not asked to log in with your Redgate ID and a record of the activation isn't created so you'll see the message "All x of the entitlements for serial xxx-xxx-xxxxxx-xxxx have already been claimed".
Manual activations will show in the customer portal as Not logged in and use up another entitlement.
To avoid running into the out of activations error you will need to remove yourself as the user in the customer portal before activating:
And then perform the Manual Activation.
You will need to repeat this each time you activate offline.