iTunes will happily import the same song multiple times if you tell it to.
In the Songs view use Shift > View > Show Exact Duplicate Items. You need to manually select all but one of each group of repeated items to remove. Don't delete from the hard drive unless you know there really are multiple files instead of multiple references to the same file.
There is also my DeDuper script which automates the process, merges ratings and playcounts and should handle the deletions properly. Please take note of the warning to backup your library before deduping. See this thread for background.
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