Activated an existing game

Bought at the Lucius sale. Then on Indigala there was a distribution of Lucius keys. I naturally saved one.
To the point - if you try to activate the key of the game on the account, which is already on the account, then Steam issues a trace. message:


Now I tried to activate the same key and it was activated! But after all, the purchased Lucius is already on the account, another copy has not appeared, and as a gift it is not in the inventory either, As there was one copy, it remained. What is this bug or feature?