Free in-game key not working

[Vurtle]

When i got the in-game key after a while when waiting in the asia server i immediately sent it to my friend as they clearly nagged me to get it for them. but when they entered it in, they encountered a message saying "duplicate product code" and a whole text wall about it. I am not sure what to do and was wondering if anyone can help.
Regards,
Vurtle