One of the most hilarious misunderstandings about evolution that creationists have is that if evolution were true, we should find animals that are mid-way between two completely unrelated species(*) such as for example something that's in-between crocodiles and ducks, or elephants and octopuses. Yet when dealing with the platypus, suddenly these exact same people make no mention of this concept, and on the contrary reverse their entire position and claim that here we have an animal that's (seemingly) impossible from an evolutionary perspective because it has features of completely unrelated species. (They like to say that "God created the platypus to laugh at evolutionists.") Talk about being two-faced... This so typical of creationist parlance; always take the position more favorable to your own views and which puts your opponent's views in bad light.
(* Of course I'm using "completely unrelated" in a more colloquial sense here. All living organisms are related to each other, meaning that every single pair of living individuals can be linked to a common parent, if we go through their lineage back enough. "Completely unrelated" would mean in this case species whose common ancestor is so old and they have diversified so much that they have extremely little in common. In other words, in taxonomic terms they belong to different orders or even classes.)
The first misconception is so stupid that it doesn't even need to be addressed. (These creationists never, ever describe the reason, the exact mechanism, that would cause such chimeric species to exist, according to evolution. They just make the claim, and that's it. No need to go to specifics.)
The second misconception is based simply on outwards appearances, with no knowledge whatsoever about the actual biology and morphology of platypuses. Just because two species might share a body part that looks similar doesn't mean that they are the same. For example many mammals and marsupials have spikes (such as the hedgehog and the echidna) that make them look similar, but that doesn't mean that they are somehow related or "in-between" the two species. Two completely unrelated species developing similar features is not even uncommon.
The snout of the platypus might have a superficial resemblance to the beak of a duck, but that's where the similarities end. It's made of completely different type of tissue, and its morphology and function is completely different (for example the duck's beak consists of two parts, the upper and the lower, which open to reveal the mouth, while the platypus' snout is one single part that doesn't open, the mouth being below it.) Saying that the platypus has a duck's beak is like saying that an elephant has a snake on its face (and therefore elephants are closely related to snakes.)
The platypus is certainly very interesting and unusual, even from an evolutionary point of view, but it's not unexpected or mysterious. In fact, the theory of evolution predicts that when species get isolated for very long periods of time, and especially if there's strong selective pressure in their environment, they will diverge significantly from their distant relative species. (It might even be more surprising that there aren't more oddities like the platypus out there.)
This raises an interesting point with respect to creationists. They are not interested in finding out what evolutionary scientists actually say about the platypus (or any other species they might conjure up as "counter-examples" of evolution.) In other words, they are not actually interested in what the theory of evolution says. They are only interested in building up a strawmanned version of evolution, which they can then easily attack and laugh at, and will ignore any counter-arguments.
Whenever you hear a creationist say "if evolution were true, then..." you just know that they are not talking about the theory of evolution, but about their straw man.