Platypusses are cool, they are like, really, really cool. And no problem for evolution at all.
First off, the beak is not like the beak of a duck. The structure of it is completely different. Also, the eggs are leather scaled, like the eggs of reptiles. Also note that the platypus is not the only egg-laying mammal. It is part of the class of monotremata, which has two other curretly living species (two species of echidna,
"spiny ant-eaters"). Now, what does it say?
Well, if you look at the evolutionary history of mammals, you'll see that mammals are descendants of reptiles. So, encountering an animal with both mammalian and reptilian characters is no big surprise. And this is exactly what a platypus is.
Man, I really should make a thread about monotremata. Maybe later.