Duplicate files on the HDD or duplicate files showing in iTunes itself?
There is an option (may not work if the names differ) in iTunes. Select "File" then "Show Duplicates".
I would usually check what you did first, because iTunes is set to "copy items to library when adding" which means anything you drag in there will be copied, and placed in the iTunes library folder. This leaves you with 2 files, one in it's original spot, and one in the iTunes library.
If you drag the same file in twice (which you may have accidentally done with ALL of them selected) it will copy it two or more times. I believe it will reference the same file in the library, but the entrys will be multiple.
Try to "show duplicates" to see if it shows them all, then try to delete each duplicate. If you still have the PC library intact, I (personally) would just delete it all and try again.
-Assuming you have iTunes copying and keeping it's library organized on the PC:
Copy the "iTunes" folder, and move it to the "Music" folder on the mac. Done. The program may be different, but the files it uses are not. Music files, XML files, library files, just put it all on the mac in the same way it was on the PC.
Hold "Option" i believe while opening iTunes, and tell it to "choose existing library" then point it to the saved library file.