PHOTO: Cancel double pics

Photos does not have a tool to search for duplicates. Are your duplicates exactly the same files, or photos at a different pixel size, like original files and smaller thumbnails or previews?

Use an application to scan the library for duplicates:

There are a few third-party tools that can detect duplicates:

• Power photos: Fat Cat Software
• Duplicate annihilator: Brattoo Propaganda Software - Products
• PhotoSweeper: http://overmacs.com/photosweeper.html

Is there anything common about the duplicates? Like all filenames ending in "_2"? Then you can try to find them with a smart album:

These applications will identify and help remove duplicate photos from a Photos Library:

• Duplicate Annihilator for Photos - $7.95
• PhotoSweeper - $9.99 - Demo version available.
• PowerPhotos - $19.95
• Duplicate Cleaner for iPhoto - Free

PowerPhotos is the iPhoto Library Manager version for Photos and is very powerful. Although more expensive I would recommend it as it has more capabilities than the others and will have added capability in the future.

Do you have the My Photo Stream enabled on your library and mobile devices? Are you also importing photos from your mobile devices into Photos via a USB cable?