There's two girls that aren't related in any way, Marta Piekarz and Cara Delevingne. In most pictures, you can tell them apart. Marta on the left, Cara on the right.
The problem is that they like making funny faces. Try telling them apart now.
I came across multiple images that I'm sure were tagged wrong. My rule of thumb is, if in doubt, anything shot by Jesse Herzog is Marta, the rest is probably Cara. This image was tagged as Cara, even though it's taken from Jesse's 500px where he was kind enough to mention it's Marta.
This is also shot by Jesse, tagged as Cara but confirmed to be Marta.
It's driving me crazy because I was trying to get as many pictures of Marta as I could, but Cara kept coming up, apparently because she's also an actress and way more famous than Marta.
Just a little rant, I don't know. You should definitely check out Jesse Herzog and his instagram. He's a genious.