How do I delete duplicate photos from my iPhoto library?

Use a third party package - iPhoto Library Manager is one good one - Duplicate annihilator is another

But the bigger question is where you are seeing these and how they got there

Are you seeing them in the iPhoto window? Or some other way?

Generally you will not have duplicates in yoru libreary as the import process gives you the options of not importing duplicates

LN

You can use one of these applications to identify and remove duplicate photos from an iPhoto Library:

• iPhoto Library Manager - $29.95
• Duplicate Cleaner for iPhoto - free
• Duplicate Annihilator - $7.95 - only app able to detect duplicate thumbnail files or faces files when one library has been imported into another with iPhoto 8 and earlier.
• PhotoSweeper - $9.95 - This app can search by comparing the image's bitmaps or histograms thus finding duplicates with different file names and dates.
• DeCloner - $19.95 - can find dupicates in iPhoto Libraries or in folders on the HD.

Happy New Year