Terms of Service and "Doppelganger Week"
If you've changed your Facebook profile photo to your look-a-like celebrity, you may be violating the Facebook terms of service agreement. Caroline McCarthy over at CNET.com writes that in the company's terms of service, it says: "You will not post content or take any action on Facebook that infringes or violates someone else's rights or otherwise violates the law...We can remove any content or information you post on Facebook if we believe that it violates this Statement." So unless you took that celebrity photo yourself or bought the rights to it, it may be in violation.
It doesn't look like Facebook is going to take any action in removing the "offending" pictures from user's accounts; but, if you're a stickler for agreements you may want to take down your famous twin's picture.