Duplicate Detection in iPhoto

When working with multiple iPhoto libraries, you can often end up in a situation where you have multiple copies of the same photo spread among multiple libraries. When using iPhoto Library Manager to copy photos to another library, either by copying albums or merging, it is often helpful to have the "Don't import duplicates" checkbox checked in the preferences so that you don't end up with multiple copies of the same photo in a particular library.

iPhoto Library Manager relies on iPhoto's built-in duplicate detection to determine which photos being imported in any given operation are duplicates. Once the duplicates are identified, iPhoto Library Manager can skip importing them a second time and optionally use the existing copies of the duplicates that are already in the library in place of the skipped photos when recreating albums and such.

iPhoto's duplicate detection, however, is not perfect, especially in version 6, and it can sometimes let some photos through even though they are duplicates of photos already in a library. There are a couple things that can be done to help iPhoto correctly identify duplicates:

Keep the filenames of your photos consistent. If a particular photo is named one way when you first import it into iPhoto, and you later import the photo again after having changed the file's name to something else, this can sometimes cause iPhoto to miss a duplicate match. Try to avoid renaming photos outside of iPhoto if this is resulting in unwanted duplicates.
Try turning off the ColorSync profile option in iPhoto (version 6 only). In the Advanced section of iPhoto's preferences window, there is a checkbox that tells iPhoto to add a ColorSync profile to any newly imported photos that don't already have one. The purpose of this is to help keep colors consistent on different devices, essentially trying to make sure that the color you see on your camera, on your computer screen, and on a printed version of a photo are all as close as possible. There is one downside to this, though, in that adding the ColorSync profile to a photo adds a small chunk of data to the photo file, making it slightly larger, and thus no longer an exact copy of the photo that was imported. This difference can in turn cause iPhoto to miss detecting the photo as a duplicate if it gets imported a second time.

If color matching is important in your photo work, then you'll probably want to keep this option enabled, but if it's not as important to you and you're having problems with unwanted duplicates in iPhoto, you might try turning this option off. Note that if you have previously imported photos that have already had the profile added, it may not help detecting duplicates of those photos, but it may help with any new photos you import into iPhoto in the future.