If that is where it came from, then either the store was sourcing their stock elsewhere and passing it off as genuine, or someone bought a genuine one and took back a counterfeit and then the counterfeit was sold to you. Presuming the store was an official Mulberry stockist, then they wouldn't handle counterfeits to know the differences - that is the reason why Mulberry (and the other brands) don't authenticate because they don't know the differences. Hence, genuine bags have been destroyed in the past as Mulberry stated them to be counterfeit and even now, we still see them repairing very obvious fakes. It is a criminal offence to knowing sell a counterfeit item, so you will need to decide the best course of action.