There is actually a tool built into iTunes made to help find duplicates in the library so you can delete them. The article you came from is a bit dated so here is the Updated version that will be relevant to your version.
How to find and remove duplicate items in your iTunes library
1. Choose File > Display Duplicates to show duplicate items (matches are based on the song name and artist). If you have multiple versions of the same song (for example, live and studio versions, or versions from different albums) you can hold the Alt or Option key (for Mac OS X) or the Shift key (for Windows) and choose File > Display Exact Duplicates. This will show only duplicate songs having the same name, artist, and album.
2. Sort by the date you added the song to iTunes by clicking the Date Added column.
3. To delete the older items: click on the item with the oldest date, hold down the Shift key, and click on the older item next to the most current date. Then delete the duplicate items by choosing Edit > Delete.
Note: If you have the same content synced with another computer, you will need to make sure that you replace the older content with the newer content in your library.
All the very best,
When deduping use Shift > View > Show Exact Duplicate Items as this is normally a more useful selection. You need to manually select all but one of each group to remove. Sorting the list by Date Added may make it easier to select the appropriate tracks. If you have multiple entries in iTunes connected to the same file on the hard drive then don't send to the recycle bin. Use my DeDuper script if you're not sure, don't want to do it by hand, or want to preserve ratings, play counts and playlist membership. See this thread for background and please take note of the warning to backup your library before deduping.
(If you don't see the menu bar press ALT to show it temporarily or CTRL+B to keep it displayed)