If that really is a copper washer, it's a problem. Copper washers are soft, used for things like sealing. If you use it on a head stud, it may compress and allow the joint to loosen.
I don't really know what is the best way to go from here. If it were me, and only one or two copper washers, I think I would just replace them, retorque, and hope for the best. If much more than that, I think I would replace them all. That means redoing the head installation, with a new gasket; a pain, but not the end of the world.