Each material comes with a number of tweakable technical and pricing parameters, and filling them out for similar materials one parameter at a time may become boring. That's where duplication comes in handy. 

Let's say you have set up Black Resin GPBK02 for your Formlabs Form 2.

You could click "Add Material" and create a new material from the beginning, 

or, you could click this icon:
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This is the result.

You can then edit the Black Resin GPBK02 copy by clicking the edit icon:  

Simply change the Material name and the Display name:
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and click "Save Settings"

The result:
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Except for Material Name and Display Name, the two materials are identical, and both are available on the widget:
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