How do I download 3500 photos from my iPhone to my iMac with out importing duplicates?

3500 photos from my iPhone 6s to my 2009 I map without importing duplicates? I am still running iPhoto's on the iMac and have already talked to Apple support and deselected the copy to photo library box in advanced settings. Most of these photos are duplicates, but there are a few I would like to import. I want to do this before upgrading to the latest OS because I am running Yosemite.