Accidentaly removed blocked device

Well, at some point I had blocked an unknow device, today I found out who it was, my grandkid's laptop and w/o checking first on how to, I, supposedly tried to allow this device back in, but due to my hurrydness AND stupidity, I removed it from the block list, LOL! Poof, gone!

Now, when I try to add it back by adding the mac address to the allow list, it says dup mac address, but it's nowhere!

HELP my kid needs her laptop!

Thank you for ANY direction!

Cheers!

Jimmy the Greek!