EA key used: true by checking only?

Hi everyone
I got a question about EA key.
I was trying to give my extra key to my friend. The key was "used: false". Before giving it to my friend i put the key to my redeem section for checking, not applying it, but it show "INVALID KEY" and changed to "used:true". Does that mean the key has been redeemed? Im so confused. How could 2 key redeemed in the same account?
Thanks for reading.