It is possible to merge candidate "duplicates." After having identified one or more identical candidates, you must:
Select two or more candidates.
Click on merge using the gray action bar at the bottom of your screen.
Choose the fields that you wish to keep.
Finalize the new file for your candidate.
Validate.
Warning: For this, the "Merge candidates" right must be activated by an administrator in the Admin/Users page for the user concerned.