The term "duplicate step" refers to a function within the automation 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. Note: If used to duplicate "Send Email" or "Send SMS", the details of the email or SMS message will not be copied, only an empty step will be duplicated to then fill in.
The purpose of this option is to save you time in building out your automation. 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.