When WE get mail with no return address that's marked with return to sender, it gets stored in our archives for a period of time and then it gets disposed of. And sometimes there's so much mail going through that you miss a couple of stamps and we never cancel them out. you don't really need to anyway, re-used stamps are quite obvious anyway, just check a stamp that's been wet and one that's new under an UV lamp.
But that's here, and we've got special shipments to Santa anyway so...
And yeah, I'd call a fake, no way the post office would custom make a stamp to use on a specific letter two weeks a year, that's not cost efficient.