When you import a skin, it is in the "Owned Skins." I believe that since there is one thing to select for the imported skin, it registers it as the same skin... This causes the imported skin to look the same. If your friend has a "Ninja" skin, and you have a "Knight" Skin, and you join your friends world, their skin will automatically change to yours, even thought that wasn't skin you imported. So basically it tries to make imported skins all look the same. I noticed this doesn't happen on servers. For some reason your friend could also perceive you wearing a "Ninja" skin even if you changed it to your own skin.