Importing duplicate photos sometimes sometimes happens. In the Photos for Mac application, users sometimes import the same photo more than once, resulting in duplicated files.
The Photos for Mac application can actually prevent duplicate photos automatically without much interference from the user as long as the photos imported into the photos still have the same metadata.
If the metadata in the photo is different then it can no longer be called a duplicate photo. For example, you export photos from a piece of software by changing its metadata. Then Photos can’t recognize it.
Tips on How to Prevent Importing Duplicate Photos in Photos for Mac
For you who get photos from mobile devices directly, such as iPhone, iPad, or iPod Touch, this duplicate photo process will usually never happen. This is because photos are uploaded directly to the iCloud Photo Library and stored there.
Unlike if you import photos from an internal drive, external driver, or from a digital camera. It may be that you will import photos that have been imported into previous Photos. As long as the metadata in both photos is still the same, Photos for Mac can recognize duplicate photos.