When I use “Duplicate To” to duplicate a file from one database to another, the file is not marked as a duplicate (not colored, and Get Info shows Instances as "0 replicants, 0 duplicates). I would expect duplicate marking to work across databases.
If I repeat this same duplication and create a second copy of the file in the destination database, both side-by-side copies in the destination are now marked as duplicates. However the original file in the first database still doesn’t show any duplicate status.
In Preferences, I’ve checked “Stricter recognition of duplicates” and “Mark duplicates and replicants in color”.
I’m running version 3.5.1 of DT3 on macOS High Sierra v10.13.6.