iphoto cleaner
There is in the app store a duplicate cleaner for iPhoto. Freeware. It will put the duplicates into the iPhoto trash. When you empty that, a folder containing those duplicates will appear in the general Trash. To be sure, save that somewhere until you are sure you still got everything in iPhoto what you want. You might have to run iPhoto first, not Photos. Then the cleaner.
I found almost a GB of duplicates in my collection