Actually i didn't suspected this one to be a duplecate one. But theoretically it could be happen. Because in the old days when i traded from a lot of different devices. I could mark it as used on one device and not on the others, then you just continue from other device where it is not marked as used. And you have a duplicate key not marked as it is and the latest updated TXT document you continue from.
Most of the suspecious keys i deleted when syncronised all the old TXT files and a bit suspicious i marked as not 100% valid.
Your key appeared to be a sneaky one.
Good luck and Have Fun!!! =^-^=