The phone book on the TomTom is a simple text file called contacts.txt in the "contacts" folder on the device.
Browse the TomTom storage memory with Windows Explorer and you can open that file with Notepad or any basic text editor.
the entries will look something like this:
"Bill ","07962104569","CELL"
"Bob","012345678","CELL"
"Brian","023456789","HOME"
"Brian","0309876","CELL"
"Chris","0165433221","HOME"
"Chris","0111222333444","WORK"
"Colin","08887776665","CELL"
Just delete the duplicate lines you don't want