It’s no surprise that duplicate management in Salesforce is not awesome. This is especially true regarding reports.
Once your matching rules and duplicate rules are setup, you normally add the Potentially Duplicates component to the lightning page. This component flags you when it detects a potential duplicate.
Additionally, you can build a report type using an object (usually Contact) and the related list Duplicate Record Items. Only then can you build a report.
However records are not automatically added to this report. You cannot even trigger a process to detect existing duplicates. Instead, it only detects duplicates when a new contact is created or updates and becomes a duplicate.
When a duplicate is detected, you can perform a Contact Merge. For some reason, this process doesn’t remove the record from the Duplicate Record Set. These are ghost duplicates and tou need to delete them manually.
The takeaway
There are many manual steps to set up duplicate management, and are many steps to manage duplicate management.
I consider it a half-hearted attempt of an important feature.