The problem is that there is no control over which "copy" is retained.
This was my belief too, for a long time, until it was pointed out that if you apply a sort to the list, then remove the dupes, it keeps the first listed and removes the rest...
Example:
All duplicated audio, sorted by Artist, Name, File size:
[Media Type]=[Audio] ~dup=[Artist],[Name] ~sort=[Artist],[Name],[File Size]-d
Now, remove duplicates by adding a final "~nodup" modifier:
[Media Type]=[Audio] ~dup=[Artist],[Name] ~sort=[Artist],[Name],[File Size]-d ~nodup=[Artist],[Name]
And see how it retains the largest sized of each group of duplicates.
Depending upon what you want, it is possible to exercise a little control over what is kept and what is removed, but you need to put a bit of thought into how to go about it.
This means that, say you wanted to keep the largest files and delete the rest, you would make a smart list using the second example, let's call that "smartlist A"
You would then create a second smartlist that included all the dupes, but excluded files in smartlist A
[Media Type]=[Audio] ~dup=[Artist],[Name] ~sort=[Artist],[Name],[File Size]-d
use the wizard to add the rule: Playlist | Is Not | Smartlist ASelect the results and send to recycle bin.
-marko