Require duplicates to be imported in Shotwell

Right now you'd have to do this at compile time by configuring with:

./configure --define=NO_DUPE_DETECTION

You should be able to build following these instructions: http://www.yorba.org/projects/shotwell/install/.

I wouldn't suspect this is a commonly used code path, so some things might not work exactly how you might expect. Please file a bug report on either pantheon photos (elementary os) or shotwell on the gnome website if this doesn't work for you.

As a workaround, I add a comment or a tag to the images already present in Shotwell. If your write the metadata to the file (option in preferences) the file changes and from then on Shotwell will import the same pictures (without the comment or tag).