Holly, Mar 27 09:26 (GMT):
Hi,
I am sorry to hear that your key is appearing as duplicate/used. All our keys are unique and have not been used elsewhere so please check that you are entering the key correctly using Capital letters where asked, as well as ensuing that there are no spaces.
Please try contacting Steam support directly as there could be an issue on their end. If they are not able to resolve this issue then please attach a screenshot of the conversation you have had with them, as well as a screenshot of the error message so we can investigate this matter further.
Regards,
Holly.
Green Man Gaming & Playfire
Customer Service