Why do I have a double face in my model and how do I fix this or join them if I have to?
This is because you have two distinct faces that occupy the same space. This is called Z-Fighting.
You can search that on the Web.
Post your skp file if possible for us to look at it.
Z fighting seems to describe it as overlapping images. Which I do not have. It’s like both pieces are broken up but it is one image. Nothing is overlapping.
Turn on hidden geometry and delete the edge that appears.