OK... you might not have to do anything as drastic as re-installing OS X.
What folders do you have 'duplicates' of and where are they sitting. For example, there are two (or more) Library folders - one sits at root level (you'll see it when you click on Macintosh HD in the Finder sidebar), one sits at each user level (when you click on your name in the sidebar). These do different things - one holds system-wide information, one holds local user info. There are a few folders like this in the system.
As far as music files go, there is a preference in iTunes to copy the music into the iTunes folder. This means that you can end up with two copies of all your songs - one in the iTunes folder (within Music) and the other the original music files that you brought over from another computer. I burned the 'originals' to data CDs and then deleted them from the Mac since they were all now in iTUnes.
If you go to System Preferences, you'll see an Accounts preference. This shows you how many users you have on your machine. There will be you (presumably as Admin) and any others you might have created (Standard or Managed). If you've accidentally created a user account you don't need, you can delete it here by highlight it and clicking on the minus (-) button.
If you really feel you have to start again (this really should be a last resort since you will lose all your files and applications!) then burn all the files you want onto various DVDs. For iTunes or iPhoto, burn the entire iTunes or iPhoto library folders, not just the music/image files inside them. Put your restore DVD into your Mac and then restart holding down the letter C. It will boot from the DVD and you should then be able to follow the instructions - once you've chosen the HD you are installing to, you may have to choose 'options' to tell it what kind of install you are doing - Erase and Install if you are getting rid of everything, Archive & Install if you are keeping your user folder/settings.