I imported many photos using the new "camera import" feature of Dropbox. Many of those were duplicates of photos previously imported by direct copying from the camera.

Strangely, the Dropbox import appears to slightly reduce the file size.

E.g. here on the right is the file imported through Dropbox:

Comparison of the two files using pdiff returns "Images are binary identical", but tools such as fdupes or even the Picasa "show duplicate files" feature, consider them as unique.

What can be the cause of this file size change? Is there any way to undo it?

Most importantly: how can I de-dupe efficiently without regard to file size comparison? (running pdiff comparison over all photo pairs in my library is obviously impractical...)

A solution for either OS X or Windows would do.