The DeDeduplicator works on exact duplicates.
It compares Subject, Date-time, Sender and Recipients info and Message ID as @Olivia_Rust wrote in her post in the following thread.
Deduplicator Support page documentation
Note: Before running the deduplicator make a manual backup first via “Menu / Backup” incase you need to restore for any reason. You can see when the backup is complete in “Show Operations” via the dropdown on the right of Refresh.