I have found that if you have duplicates on your phone you also have it in itunes. I delete them from itunes and then that deletes it from my phone.
Are you manually syncing your iPhone (manually choosing what songs to put on your iPhone) or does it auto-sync from your library? If you are auto-syncing then you either need to delete the duplicates from your iTunes library or you need to uncheck the tracks you don't want to sync. If you are manually syncing your iPhone then you need to manually delete the tracks you don't want on your iPhone while it is connected to iTunes. Make sure to select the songs from the "Music" list and not from a created play list or they will not delete from the iPhone (just from the play list). If this is confusing you might be able to better understand by looking at the Apple Support site. The following link explains manually managing your iPhone (or iPod)...you can find info about auto-managing and everything else on this site also. http://support.apple.com/kb/ht1535
I would like to thank the members of the community for their correct responses. I have provided additional instructions directly from the www.Apple.com website under the (support Tab). I have also provided the link directly to the page below:
You may sometimes see an item displayed more than once in your iTunes library. To find and remove all items that show up more than once in your library, follow the steps below.
iTunes 9, iTunes 8 for Mac, iTunes 7 for Windows, iTunes 8 for Windows, iTunes 7 for Mac OS X
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.
You can connect your iPhone to your MacBook. Then download third-party software to scan for duplicate files. You can use iMyMac's Mac Cleaner. It's lighter and safer.