I saved the very best (and closest comparison) for last this time, so make sure to scroll to the bottom of the page.
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On the right: Jamie Campbell Bower
May as well start with a wrestling-related one. It seems those pro wrestlers have doppelgängers all over. I'd mentioned before that Wilson Bethel resembles Edge. I still stand by that statement.
However, I think Jamie Campbell Bower looks even more like Edge. Jamie Bower, for those who are unaware like I was, is an actor who has quite an impressive resume already.
I just watched The Mortal Instruments: City of Bones (review to come later) and that's the first time I saw him...or so I thought. Jamie was in the final three Twilight movies, Harry Potter and the Deathly Hallows: Parts 1 and 2, and the Sweeney Todd movie that starred Johnny Depp.
The hairstyle, obviously, has a lot to do with the resemblance. However, I think the bone structure is there too. I'm not expert on the matter, but that's the same exact chin there, a similar nose and the same shaped head. A solid celebrity look-a-like for sure.
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On the right: Katy Perry
This pair doesn't have similar bone structure. In this case, I think a lot of the similarity is in the hair style and color. They don't have the same color eyes and Katy Perry's face is more full.
Yet, if you take a quick look at them, one could probably pass for the other.
The only thing I can figure is that it's the hair that does it. It probably doesn't hurt that there aren't that many Hollywood ladies with jet-black hair, so automatically kind of looks like the other.
Outside of that lady on N.C.I.S., I can't think of any others.