I've seen this come up several times before in costumer circles, and the general consensus is that it is indeed OK to use the same inspiration image as another costumer--for example, I know of at least three costumers who have completed/will complete the Marie Antoinette riding habit from Fastes de Cour. Plain and simple, we are not designing something original when we make a costume, we are simulating something that had already been done. We are all copycats already!