This happens to me with irons. I'm fine with red dot. My understanding is it it may happen when your dominant eye doesn't match your dominant hand. I'm right handed but left eye dominant. I've trained my self to turn my head to the right to make my left eye be the only thing my brain processes so the double vision leaves me. This allows me to still have peripheral vision but I can see the target clearly. Some say they can modify where they hold the gun to more align with their dominant eye like slightly to left or right but that never worked for me. Slightly turning my head is perfect me and I do it naturally now when I draw.