This could happen if the same folder is monitored twice by WMP, for example, if both C:\Media and C:\Media\Music are in WMP's list of monitored folders.
To view the list of monitored folders, press F3 in WMP, and make sure that the Advanced Options are visible. What folders are in that list?
Another possibility is that WMP's media library got corrupted. To reset it, follow the instructions for WMP 11 on Vista at http://zachd.com/pss/pss.html#medialibrary . Note that resetting
the library will make you lose all current play counts, and possibly ratings too.