Already Activated.

Yeah

Write a comment in your win that it's already activated, so the other person can see it.

If I were in your position, I would also agree to cancel the giveaway if the gifter can't provide another key.

I would let the gifter know the key didnt work first before deciding by myself.

Give the giveaway creator a chance to replace the faulty key by messaging them on their giveaway. If they have not got a replacement, then they may ask for you to agree to delete the giveaway. If they have tried to replace the key, i would agree to deletion but you do have the option to mark as not received if you want.

First, comment in the giveaway so the gifter sees that the key doesn't work, then if he can't provide a new one, let him delete a giveaway. No harm done that way, and you will get your points back.

write the gifter, had it once that my key didn't work because i got a diplicated key from gmg. wrote the support, got a replacement key and so got the gifted.