UV symmetry?

[mireazma]

Hi there. I marked edges for cut and then unfolded a symmetrical obj. It turned up pretty good with left and right sides symmetrically placed on the UV map as expected. There were some barely visible differences though. When I mirrored one island and placed over the other they didn't match perfectly. If I hadn't mirrored and overlap them I could swear they were perfectly symmetrical.
Question: does anyone know of a trick to make them fit 100%?