Duplicate top of selection stack is a logical operator. It is used to duplicate the top of the current Action list stack. It's typically combined with the Remove top of selection stack ("delete") operator.
Both "Duplicate top of selection stack" and Remove top of selection stack are only used internally by PitStop.
When running a Preflight Profile, PitStop turns this Profile into an Action List; the more checks and fixes there are in the Preflight Profile, the longer and more elaborate its Action List representation. In order to avoid having to perform complex selections multiple times, PitStop optimizes the internal Action List by duplicating these selections when they are needed multiple times.