Add Ability to 'Add More' When Editing a Block

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Okie dokie then... I have a very simple block I'm working on where the user enters a couple basic text fields. For example, one is a category title, and then one is a sub-category.
I need to be able to make it so that the user can add a 'sub-title' (as many as they want). We'll call this 'Group 1'.

THEN.. they need to be able to add another 'group', but with the same fields.

What's the best way to go about doing this?

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