Each material comes with quite a number of tweakable technical and pricing parameters, and filling them our for similar materials one parameter at the time may become boring. That's where duplication comes in handy.
Let's say you have set up Black Resin GPBK02 / Black Plastic for your Formlabs Form 2.
You could click "Add New Material" and start with Grey Resin from the top,
, you could also instead click this icon:
This is the result. You can then edit the Black Resin GPBK02 copy / Black Plastic by clicking the edit icon:
Simply change the Material name and the Display name:
and click "Save"
Except for Material Name and Display Name, the two materials are identical, and both available on the widget: