I don't like dupes because they tend to phase the audio and reduce its quality
Select clip in timeline, go to toolbar > audio filters > parametric eq. (i'm going off memory here). Move the slider to about 100 hz and drop the gain. (A fun way to play with it is to crank the gain to identify where the problem eq is, then drop it.)
Also- see apple soundtrack pro's website. It has demo's on how to easily clean up audio beyond what fcp can do, and it is a lifesaver when bad audio happens.
Good luck!