Hello there CalamityJayne942,
If you have duplicate items in your library you should be able to delete the extras without issue. There is actually a tool to help get rid of duplicates in the library.
How to find and remove duplicate items in your iTunes library
http://support.apple.com/kb/ht2905
The article says that the function is under the File menu, but in the newer versions of software it is in the Vew menu.
Cheers,
Sterling