Duplicating extracting rules is especially useful when you need to create a complex extracting rule while you already have a very similar existing rule. So, you don't have to go through all the filters and create a new extracting rule, but instead duplicate the existing one and tweak it based on its requirements.
In order to duplicate an extracting rule go to extracting rules by clicking on 'Extracting Rules' button of the relevant extractor. Then hover by your mouse over the extracting rule you want to duplicate and you will see that 'Duplicate' button will appear. After clicking on it, the following window will open where you need to enter a name for the new extracting rule and specify the extractor in which a copy of the rule will be created.
The following video episode demonstrates duplicating an extracting rule.