Merging Photos Libraries without duplicates

When you have that library open, go to photos preferences, and click the button "use as system photo library"

The only way to do it with Apple's own tools is to make all your libraries your iCloud Photos Library in turn, one after the other. Each library will be uploaded into iCloud Photos, and iCloud will merge the libraries, and solve conflicts between duplicates. If you have exact duplicates across the libraries, iCloud will pick one of them as a keeper.

It is the only way to merge the libraries in a lossless way, while preserving all albums, folders, metadata, faces, places, and keep the edited versions paired with the originals, so you can revert all adjustments individually.

You will need enough iCloud storage to hold the merged libraries and a lot of patience, because uploading a large library can take several days.

On top of what Leonie said:

The "exact" duplicates is important. This method can only avoid duplicates when the files in the different libraries are from identical originals. If the original filename is different, if the filesize is different, if the metadata is different, if they've been edited or resized before import into photos, they won't be recognised as duplicates.

And if you already have duplicates within a single library - this process won't remove them.