There was the same question some time ago. For some reason its a bit more complicated to handle duplicates within MJ as it is in MC. They removed the ability to create smartlists the very easy way in MJ via import/export.
To create a new smartlist right click "Playlists" and select "Add Smartlist". A window pops up, where you can set the smartlist options.
Here I did some Screenshots to show you, how to set up some duplicate-searching smartlists. You only need the second red marked entries of each smartlist, the rest is optional, may be diffrent for you and doesn't effect the function of the lists.
- the first smartlist finds all your files with the same filename without looking at the filepath
- the second one finds all files with the same artist and title tag
- the third offers some kind of fingerprint search. This only works for audio files which have been analayzed within MC/MJ and for some reason it doesn't work perfect as I reported here (http://yabb.jriver.com/interact/index.php?topic=46948.msg322207#msg322207).
(http://www.pix01.com/gallery/BD01FC74-5847-4306-A29B-3AA1BF99BB82/MC_Screenshot_._Smartlists_to_Remove_Duplicates/182151297_orig0.jpg)