Training
As noted above, two images are normal.....they are also an indicator that your eyes have already begun to adapt to one eye focus. Note what happens when you lower the gun away from the aiming path as you are seeing two images.....as soon as you lower the gun, you see one image. What happened? Well, you removed the object that you were supposed to focus on....the front sight....and your two eyes reverted to normal vision.
It is helpful to remember when you are training the eye for both open shooting that your aiming eye is not focusing on the target but on the front sight which is less than three feet away.
The human eye is remarkable for its adaptability. The best example of this is the fact that the images that the lense of the eye focuses on the retina are upside down.....the brain flips them for us so we perceive them as upright. Compared to that, training to focus attention on the front sight while ignoring the image from the non aiming eye is no big deal.