Helium Music Manager comes with a very easy to use tool for locating duplicates in your library.
It will scan your entire music library for duplicates in a few seconds. A list of all duplicates is presented which lets the user either remove all duplicates automatically or manually chosing which copies to leave and which to remove.
To automatically select all duplicates, click the Select all duplicates menu option.
This will select all duplicates, prioritizing to keep the tracks with the best quality (bitrate, size and file type is taken into account).
When the duplicates are selected, just click the Remove button to remove the files from your computer.