Extra Dust Jacket

Recently bought this 1st/1st of Tree and Leaf and after paying the seller told me that he had found a second jacket underneath the main one. He generously included it without charging extra. The jacket on top is worn with age but the one underneath looks brand new, having been well protected. I was just wondering, is it likely that this happened accidentally when the book was first jacketed for sale? Having never seen it myself, I'm curious if that sort of thing is common or not.