Are these deleted just in the playlists or within the main content of your library?
For regular playlists just delete the extra tracks you added.
If you've accidentally duplicated all or part of your library... then I've written a script called DeDuper which can remove unwanted duplicates automatically. In addition to making sure your library ends up with a single copy of each duplicated track it also merges playcounts & ratings and ensures the remaining songs become connected to any playlists that the duplicates were part of.
See this thread for background where I discuss the different types of duplicates, the manual approach to removing them, and the thinking behind the genesis of the script.