You are using Case Merge Premium and want to ensure you're using effective duplicate search criteria.
Case Merge Premium will reference your duplicate search criteria as users interact with the Duplicate Detector, Duplicate Case Utility Bar, and Automerge. Administrators can define unique duplicate criteria for the organization as a whole, by Record Type, and Automerge.
Upon install, the organization-wide duplicate criteria we package is configured as follows, searching for open cases with a similar subject.
After some time of using Case Merge Premium to resolve duplicates, we recommend reviewing the Criteria Performance section of the setup experience. As merges occur, Case Merge Premium will gather information on how well your defined duplicate criteria is performing, and also offer suggestions on other fields that may be worth adding to your criteria for better duplicate matching.
In the example below, we can see that 102 merges have been performed over the last 60 days. The criteria we're currently utilizing is performing well. However, based on the suggested criteria, the search criteria should be updated to also factor in fields such as Account Id, Business Hours Id, Case Reason, and Priority.