I did not do anything when iPhoto migrated to photo on my iMac. I now have 3 photo libraries taking more than 550Gb. I need to make some space on my HD and I suspect some photos have been duplicated. How can I merge the libraries and delete automatically duplicate
[Re-Titled by Moderator]
Original Title: 3 photo libraries appeared on my iMac after migration iPhoto to photo How to merge?
iMac, OS X 10.11
Can you upgrade your Mac to macOS 12 Monterey? With macOS 12 Monterey we can merge Photos Libraies by importing photos directly from one Library into another Library. Photos will import edited photos losslessly, by keeping the originals stacked with the edited versions as one phot and we can revert adjustments individually. However the keywords and albums will not be imported, see: Import photos from another library in Photos on Mac - Apple Support
On older system versions you could use the third-party app PowerPhotos (the full, paid version) to merge the libraries.
Or, if you are using iCloud Photos you can merge the libraries in iCloud. Make each of your Photos Libraries your iCloud Photos Library in turn and wait for the photos to upload. Photos will merge the libraries with all albums in iCloud and solve conflicts between duplicates.
Otherwise you would to have to export the photos from each library and import them into your main Photos Library.
On the more recent system versions the Photos.app is much better than the early versions on OS X 10.11. Try to get at least to macOS 10.14 Mojave, the last version, where you could use iPhoto and Photos simultaneously and compare the new Photos Library to the iPhoto Library side-by-side.
Merging multiple photo libraries and removing duplicates on iMac
