"I happen to know for a fact that she was an American version of me. She was signed by my label in America as, 'We need to find something controversial and kooky like Lily Allen.'" —Lily Allen on Katy Perry
This explains a lot. I don't see the similarity all that much, unless I put on my major label executive goggles, in which case it makes perfect sense. Those are also the goggles that make Duffy look like Amy Winehouse.
Incidentally, does anyone know why Lily Allen talks so much shit on her label? I love her first record, and her second one should be pretty good (I like the first video), but as far as I'm concerned she should be thanking her lucky stars (and MySpace, and Mark Ronson) that she got signed to a deal in the first place.