Symptom
There are duplicate pictures in the iPhoto Library.
Solution
Make sure the date and time settings on the camera used to shoot the pictures are correct. See your camera's documentation for information on how to set the date and time.
iPhoto saves pictures to the iPhoto Library in files and folders based on the date and time each file was created. iPhoto alerts you to possible duplicate files if it tries to copy a picture into a library folder that contains a photo with the same file name.
If the date and time settings on the camera used to shoot the pictures are incorrect, iPhoto may use the date when you import the pictures, which may result in duplicate pictures.