I say "KAA-ren". I'm sure there are some Americans who say KAIR-in and KEHR-en (imagine that an upside down e, I don't know how to type that.) But I've never heard anyone put the stress on the second syllable in
Karen, as Americans do with
Corinne, and that makes a huge difference, I think. I would never mishear
Karen for
Corinne or vice versa.