This is simple but makes us to think, so you want to make this effect. Then you are in right place. For this effect just select image from which face can be selected.
First you need to select face i.e. eyes, mouth and nose area.
Then, you must need to duplicate / Copy & Paste that selection.
After this step you need to transform by going Edit -> Transform -> Flip Horizontal
Now, you will see similar to photo but still we need to work more.
After this, we will add Layer Mask to selection layer and clear sharp / irregular edges.
Select all layers and Convert to Smart Object.
Thus, we are about to complete the effect.
Now, just again go to Edit -> Transform -> Flip vertical.
And done your image is ready.
