It doesn't hurt to have a double degree, since it means you invested more time and effort into your education, are probably more well-rounded academically, and challenged yourself. But at the same time, I don't think it helps a whole lot either.
I, myself, received a double degree from the University of Washington. A BA in Biochemistry from the College of Arts & Sciences, and a BA in Earth & Space Sciences from the College of the Environment.
However, my GPA is poor by medical application standards and retrospectively I would probably have been better served not aiming for the double degree, and instead taken less credits and less rigorous courses to boost my GPA.
So to sum it up, having a double degree does look impressive on paper and it does help your case a bit, but not to the degree that something like a high GPA or high MCAT would.