Comment 3 for bug 318694

What is happening is gthumb ships 2 desktop files. One of them is to launch gthumb normally, and the other one is to launch gthumb with "--import". They both specify different mimetypes though, so you should be presented with the correct gthumb launcher when you right-click on an image. The desktop file that launches it with "--import" specifies x-content mimetypes and is used to launch gthumb when you insert removable media containing images.

What is happening though is when you right-click on a folder, Nautilus presents both gthumb launchers in the list of applications to choose from (so gthumb will appear twice). A workaround is to select the other gthumb launcher from the list.

I don't think this is a gthumb bug. I shall re-assign back to nautilus for now.