Apple Photos duplicates

I am seeing a duplicates album in all Photos Libraries, if there are duplicates. But only, if the libraries are on a volume, that is always available for the background processes, that are scanning the library. And it may take weeks for the duplicates album to appear in any library (depending on the size of the library). The system Photos Library is special, and some features are only available for the System Photos Library. Other applications can only access the Photos in the System Photos Library, and only the System Photos Library can be used with iCloud Photos, but the other Photos services are available in all your libraries. See:

System Photo Library overview

Create additional libraries

You're welcome, Greg 😊