Should I tell seller no?

[Antonio Bodley]

I had a seller ask me, "When a home is under contract can the seller still attempt to sell?" I was going to tell the seller no it's not possible to do if it is under contract. Of course I don't want the seller to sell the property without me. That would take profit away from me. If a seller wants to take matters into their own hands and sell their house while cutting me out of the deal, should I allow this to happen if they find a buyer on their own before I do?