You can 'Factory reset' Duplicate Cleaner one of two ways. Note that doing this will cause the current scan results and any cached file hashes or metrics to be erased.
Method 1 (Version 5)
In the settings tab, select the General subtab and click on the 'Reset all settings and data' button. The program will close and restart.
Method 2 (Version 4)
If you are using version 4 you can access the 'Reset all settings and data' button in the Advanced Settings tab.
You can delete the database file using Windows Explorer. Close Duplicate Cleaner first.
The database is located in:
The file will be called DuplicateCleaner_Pro.data, DuplicateCleaner4_Pro.data or DuplicateCleaner.data, depending on the version you are using. Note that the AppData folder may be hidden - you'll need to enable 'View hidden files' in Windows Explorer.