For those unsure which eye is dominant- with both eyes open, look at an object 20 ft away. Keeping both eyes open, raise your extended arm, with your thumb up (like hitchhiking). Position your thumb so it blocks the item you are looking at. Without moving your hand, close one eye. Open it, close the other. The eye that when open does NOT move the object side to side is your dominant eye.
Most folks have troubles trying to focus on rear sight, front sight, target- they are at 3 different focal lengths. Try focusing on rear, THEN front, THEN target. Target will be sharp, front a bit fuzzy, rear real fuzzy.
If your double vision problem is TOO distracting, get an inexpensive pair of shooting glasses (yellow lens safety glasses, Home Depot) and cover the non-dominant lens with tape or carboard.
But DO try to focus on the target, both eyes. Of course, scopes are a different matter.
Think Doc Holiday had a different solution in Tombstone, tho.....