Curing the Double vision issue.
If you are a right handed, right eye dominant shooter, there are not many posts to help you correct this problem. I can tell you what worked for me, and try to explain my sight picture.
Relax your eyes and allow them to go slightly out of focus. To better explain this, try to focus as if you were looking at one of those 3- dimensional photos you see at the mall, you know the ones that have a hidden picture of a schooner or a dolphin.
My sight picture has the target slightly out of focus, front sight in focus but not as sharply as other members here describe. I think the ideal focus is one just good enough to distinguish spatially where the front sight is on the target. I see through my right hand and the rear sight as if it were superimposed on the target.
I think the doubling comes from focusing too hard on the front sight which makes you go cross eyed resulting in a doubled target. Try to think of it this way, you are not focusing on the front sight, just looking at the entire picture in your field of vision, the gun and your hand just happen to be there.
It took me about 600 rounds to figure out my proper sight picture. Once I started using this focus my groups at 12 yards dropped from 3 inches down to 1 inch and change, and I am now ready to move the targets back. This sight picture and focus also cured my slight left shot push, I believe I was getting confused about which double of the target to shoot. Also, I can now call my shots accurately. When the target doubles it is nearly impossible to figure out where on the target the front sight is at moment of recoil.
What is the cause of double vision? I have no idea, but I would be interested in knowing if any other shooters that double up have one eye with slightly better vision over the other. I am 20/20 in the left and and slightly worse than 20/20 in the right but do not wear glasses.
Hope this helps