Not a stupid question at all. As I see it, shallots sets have been grown from seed, so they are a first generation plant. When planted they will grow to produce the familiar clusters of shallot bulbs/cloves. These bulbs can be replanted to continue the cycle and produce more shallots. What I cannot say though, is whether these bulbs will have the same vigour as the original sets, as I have not been successful with replanting ... However if you choose large bulbs, then these should be stronger as Robert says ...