I think you can select the one you want by touching it on the touch screen. I'm not 100% sure of that.
This mostly happens to me when I'm shooting events. Inevitably it wants to focus on a different face than what I want. I'm not sure if it goes for largest/nearest face, or brightest/highest contrast - but I'm normally shooting a primary subject surrounded by a crowd, with the subject often in profile, so it'll chose someone in the crowd to focus on. Because of this I turn off face AF when shooting events.
I recall my EM1.1 would focus on the largest face. This was a problem when I was shooting mothers holding their babies (the camera wanted to focus on the mom 100% of the time, and I wanted the baby focused 100% of the time). Again, I've just stopped used face AF if there is more than one person in the photo.