The story does not make clear whether Beauvais had seen the arrangement of flowers on the living Marie's hat, or just knew that she used to wear these approximate kind of flowers. If the former, finding all the flowers together is certainly much stronger evidence that the corpse is Marie than finding just one (even though they are not strictly independent). If the latter, then Dupin's reasoning indeed seems fallacious on this particular point, though not on the more general one of whether the identification of the corpse is beyond reasonable doubt.