Duplicate songs in iTunes can be a real pain in the behind. If for some reason you find that you have duplicate tracks in your iTunes library, you can easily remove these manually with the following process:
View Your Duplicates
In the iTunes "View" drop-down menu, select "Show Duplicate Items" option
This will display every song in your iTunes library that you have multiple copies of.
Isolate Your Duplicates
If you you do not have it set up already, you will need to turn on the Date Modified or Date Added column in iTunes. To do this, just right click (or command click, or two finger tap) on the blank space to the right of the last column title ("Plays" in the example below), or left of the first column title (the "Tick" symbol) and select "Date Modified" or "Date Added"
You will then need to click on this column title to sort the duplicates according to the date they were added or modified.
Delete Your Duplicates
Once the list is sorted, select the recently added songs and hit the delete key.
Then confirm that you want to delete this selection...
... and confirm that you want to move the files to the Trash (or the Recycle Bin on Windows)
And Voila! Enjoy your new "Duplicate Free" iTunes library.