iPhoto on you computer does not have a way to look for duplicates within the application, as it is designed to read the meta data attached to the images, not recognize image similarities
You can take a look at some of these 3rd party applications that are designed to help you locate and eliminate duplicates.
1. iPhoto Library Manager - http://www.fatcatsoftware.com/iplm/.
2. iPhoto Duplicate Cleaner - http://iphotoduplicatecleaner.com.
3. Gemini: The duplicate finder - https://itunes.apple.com/us/app/gemini-the-duplicate-finder/id463541543?mt=12.
- support phone no. - 1 877 562 2729. Works for more than just iPhoto (can look for duplicates files as well).
4. Duplicate Annihilator - http://duplicate-annihilator.en.softonic.com/mac. and http://www.maclife.com/article/howtos/how_get_rid_duplicates_iphoto.
5. Decloner - http://www.pixelespressoapps.com/decloner/.
And iPhoto Library Manager - http://www.fatcatsoftware.com/iplm/ - will merge your libraries too - the only other wayu to merge librares is to use a unified libraruy which requires iPoto '11 version 9.3 and Aperture 3.3 or newer