The Duplicates album is only available in macOS 13. Your system version macOS 10.15 Catalina is too old. You need to upgrade to macOS 13 Ventura to be shown the duplicates in a separate album. Is your Mac compatible with Ventura? macOS Ventura - New Features - Apple (UK)
Ironically, Photos 5 on Catalina is spending a lot of time on searching for duplicates, but it is using the search result just to created a pleasing, curated layout of the photos by omitting the duplicates. You could use the Moments to as a hint to find Duplicates: About the Duplicate Detection in Photos 5 - Apple Community
To find the duplicate HEICs imported from the iPhone in one place look at the "Imports" album. There the photos from one Imports session should appear all together. Or use a smart album:
- File > New Smart album
- Set the rule to "filename ends with ".heic"
- Add a rule "date added is ...." for the day you imported the duplicates
- Add a rule "camera model is ..." for your iPhone
- Combine all three rules with "Match all".
This smart album will show you the recently imported heic files together.
You may want to keep the heic files instead of the JPEG versions because of the better compression, as the jpegs are needing twice as much storage. And as your iPhone is set to use the HEIC format, you are risking to import duplicates overland over again, as long as you are importing the photos from the iPhone concerted to JPEGs.
