Whenever I copy my mail files, e.g. Inbox, Sent, etc., I don't copy the equivalent .msf, or "header" files. Those files, which are found in the Profile folder, should not be copied in first place, allowing the program to create new header files, or should be deleted, with the program closed, and, then allowing it to create new header files. Typically, duplicate mail is caused by corruption of the .msf files. The solution is to delete the existing .msf files, with TB closed, and then restarting TB. I never saw duplicate mail if the .msf files were deleted.