The term "duplicate step" refers to a function within the drip campaign designer that allows you to copy a previous step in the workflow.
Duplicate step can be used for Flag Event (the Flag Event Step only will be copied, not the entire branch after it), Pause, Lead Alert, Wait for Event, Update Field, Export CSV. You cannot duplicate a mailing or SMS message.
The purpose of this option is to save you time in building out your drips. In the example below, we have a 6 day pause we want to duplicate. To achieve this, we simply pull the "Duplicate Step" element onto the workflow step (the pause) we want to duplicate.