If these are purchased items, and one of the tracks of each pair has a cloud download symbol, then try signing out of the iTunes store and back in again to resolve. If you still have cloudy duplicates you may want to remove the downloaded item and then download the cloud item or hide the purchased item from your purchase history.
iTunes may create duplicates if the same content is repeatedly added from outside the media folder when it is set to make copies of anything that is added to the library, or is added from an external drive that hosts the media folder if it was offline when iTunes was launched.
Apple's former advice on duplicates is here: Find and remove duplicate items in your iTunes library (archive copy). It is a manual process and the article fails to explain some of the potential pitfalls such as lost ratings and playlist membership, or that sometimes the same file can be represented by multiple entries in the library and that deleting one and recycling/trashing the file will break any others. On a Mac there may be added confusion as iTunes might still be able to play files in the trash until it is emptied.
When trying to identify duplicates start with File > Library > Show Duplicate Items and then click Same Album to display exact duplicates as this is normally a more useful selection. You need to manually select all but one of each group to remove. Sorting the list by Date Added may make it easier to select the appropriate tracks particularly if used immediately after the dupes have been created. If you have multiple entries in iTunes connected to the same file on the hard drive then don't send to the recycle bin/trash. If you don't see duplicates that you expect to see then it may be that minor differences exist such as a disc count of 1 in one copy of album which has a blank disc count in another, or there are two alternate versions of a band's name. Ensuring that you tag your albums in a consistent fashion will make it easier to spot and then remove duplicates.
Mac users can try Dupin or Dupin Lite 2 from Doug's AppleScripts for iTunes.
tt2
