ITunes duplicating songs?

“When I add my media folder to iTunes, it converts and duplicates many of them, but not all.”
Go to the Settings/Preferences Menu (or File/Preferences Meuif on a Mac)/Advanced: uncheck “Copy files to iTunes Media folder when adding to library”

“They are all mp3 files.”
Settings (or Preferences if on a Mac)/General: Under the setting “When you insert a CD” there is a pull-down menu next to it, and to the right of the pull-down, is a button labeled “Import Settings;” Click this button and a pop-up window appears-“Import Using…” and a pull-down, should be set to .mp3; you can change to .AAC, .AIF or .WAV (the last two choices are “uncompressed” and take up more room than .mp3 or .AAC

“There are no playlists.”
Importing media only imports the raw file (song) data, not your playlists. If you have previously created playlists from another (previous) iTunes Library, the only way to retrieve those playlists is to find an open the “iTunes Music Library.xml” file from the last library you had (There is a master folder called “iTunes Folder” and inside that folder you’ll find that file.) You would need to copy that file into the new iTunes Folder. If you have wiped the previous iTunes Folder then you won’t be able to recover those old playlists and you’ll need to rebuild them.

”…and it does not appear that an actual converted duplicate file exists because there is no more space missing on my drive, but most of the songs appear twice.”
Okay on that one, it is possible to import the “link” to the file more than once, if you are importing media from the same location more than once (for instance, going to the File/Add to Library command and importing your media folder more than once will do this).

To remedy this, you can go to the main library in iTunes (in the main window, on the left-hand column, at the very top of the list under “Library” click on the “Music” button to display your entire music library. Next, go to the File/Show Duplicates (in previous versions of iTunes it may be under a different menu like the “Advanced” menu). Then somewhere (either in the iTunes window or in the menu) there should be a command to “Remove Duplicates.”

Another way to do this is to sort by Date Added (View/View Options) and make sure that the “Date Added” box is checked). Then in your main iTunes window, click on the top bar labeled “Date Added” and the library sorts according to the add date/time. If all goes according to plan you should be able to see two (or more) different import dates/times. To select a range of files click on the first one in the list, then while holding down the shift key, scroll down the list to the last file you want to select and click it. Then hit the “Delete” key or Edit/Delete.

Hope that helps.