The answer depends on if the duplicates are on the hard drive or just in the library.
If the duplicates are just in the library and pointing to missing files, run Auto-Import and tell it to fix broken links.
If the duplicates are actually on disk, you can use Smartlists to help you out.
As a starting point, create a new Smartlist, then use the Import / Export button and enter this:
[Media Type]=[Audio] ~dup=[Album],[Artist],[Name] ~nodup=[Album],[Artist],[Name]
That first gets a list of possible duplicates, then removes all but one of the copies. You can delete everything in it. Then refresh. It should be empty once you have no duplicates.