Don't know much about either of them, but from what little I've seen of them, they've styled themselves the same way often enough for it to be understandable to mistake one for the other.
Clothes/hair/fashion-wise, I mean. I couldn't tell you anything about their music and if they sound alike.
I still can't reliably tell Jessica Chastain from Bryce Dallas Howard. utee posted a funny video about that a while back.