I thought I'd start a series of posts on hidden or overlooked features in Hazel. Most should be documented in the Help but they still get overlooked.
So, onto our tip:
If you want to duplicate a rule quickly, select the rule you want to duplicate and while holding down the option-key, click on the plus button (the one that adds a new rule). When you hold down option, the plus button will change to a double-plus to indicate that it will now duplicate instead of just add.
Note that this functions identically to the "Duplicate" item in the action/gear menu.