Jack,
I too noticed that duplicate issue after using SuperDuper (SD). So I asked Dave Nanian, the developer, if he had insight. His reply:
"The reason you see multiple items, after using SuperDuper!, is because you're creating new copies of the applications (on the backup). Since the paths to these applications are different, LaunchServices puts them in again."
I presume this is also the case with Carbon Copy Cloner (CCC).
As Bill noted, rebuilding the Launch Services Database with Cocktail (or similar utilities) is a solution.
Alas, if you are not using SD or CCC, I know not why you are creating the duplicates.
But if you are using SD or CCC, you may well need/want to rebuild the Launch Services database from time to time as the condition will recur. OTOH, I know of no adverse consequence from ignoring the condition.