In the Pokémon anime, the trio of Jesse, James and Meowth are constantly putting on various disguises in order to trick Ash and his friends, and everyone always falls for these often paper-thin masquerades every single time no matter how many times it happens. Most fans seem to point to this as the result of Ash and his various companions being extremely gullible and unobservant to fall for the same trick over and over again. But consider the fact that the Pokémon world does not work the same way that the real world does. In the real world, almost everyone has a distinct appearance and the chances of running into someone who looks exactly like someone else are pretty slim (unless they have a twin or something). But in the Pokémon world, at least in the anime, doppelgangers are much more common. There are hundreds if not thousands of Nurse Joys and Officer Jennys running around, countless background extras have their designs recycles, several of Ash's companions have run into their exact duplicates, Ash himself used to constantly be mistaken for other kids that look like him and of course the main characters have met at least one Jessie lookalike in the form of Jessebelle. Sure, Ash and company get tricked by Team Rocket's disguises on a near weekly basis, but they're the optimistic and friendly types who are willing to give people the benefit of the doubt. Just because these two random strangers they run into look like Jessie and James doesn't mean they are Team Rocket and until proven otherwise Ash and "the twerps" aren't willing to just assume that everyone they come across are enemies when they could just as reasonably (in-universe) be random look-alikes.