OffShoot will automatically detect duplicates when copying data into an existing folder structure. A file will be not be copied when all of these conditions are met:
A file with an identical name exists in the folder the copy will end up
That file has the same byte size
That file has the same modification date
When a file with an identical name already exists on the destination path, but has a different byte size and/or modification date, it will be copied using a counter following Finder and Explorer's duplicate naming conventions: B002C006_180327_R1ZA.mov becomes B002C006_180327_R1ZA 2.mov.
Skipped files will be logged in the Transfer Log, under the section:
-- Skipped files (already on destination) --
Duplicates that do not match all criteria are logged as such:
If an identical file exists in a different folder on your destination, the source file will be copied. If you are looking for a way to find duplicates on an existing drive, have a look at Duplicate Detective or Gemini.