The closer your dominance, and the more target focus you have, the more likely this is to happen. If you focus hard on the target, you may want to read what is written about front sight focus and shot calling here on the forum.
With practice, and with a consistent presentation to the target, it will get easier to ignore the second image.
The more you focus on the front sight, and the faster you pick it up, the easier it gets.
If you cannot pick up on it, try a piece of clear tape in the lens of your non-dominant eye for a while. This forces the focus on the dominant eye to increase, and the second image goes away. Then, over time, make the tape smaller and eventually remove it.
I have always shot both eyes open on pistols, rifles, shotguns, even magnified optics. It is not essential to success, but it is much easier if you can do it.