Posting Photos in Additional, Exterior, and Interior Categories. And Duplicates

Does the image from a post also go to one of the photo locations (additional, interior, exterior)? Does it matter if there are duplicate photos?

No, if you want to publish an image to the photo section of your Google Business Profile you will need to use the photo tool to optimize an image and schedule it for publishing. It doesn't matter from a rank perspective if there are duplicate images, but it can look unprofessional.