Before you decide to back up your iTunes library to mSpot or Dropbox, it would be a good idea to get rid of any duplicate files. If you use the File>Show Duplicates menu choice, the generated list may show tracks that are the same songs yet produced or mixed differently, such as live cuts, studio cuts, or extended versions. It is much more helpful to use the File>Show Exact Duplicates menu option. To access the filter, hold down the option key on the Mac OS, or the Shift key for Windows.
For example, in my library, the standard listing of duplicates resulted in 727 songs taking up 3.64 GB. The exact duplicate list revealed 147 items using 623.4 MB. I much rather deal with sifting through the latter.