Indeed Sallyanne is right, and there is no button straight out of the box in these 2 dialogs to duplicate a brush as well.
To duplicate a brush to be able to edit it, you need to go to the brush dialog (Windows > Dockable dialogs > Brush), then here you can duplicate it then edit your duplicate.
To duplicate a brush to be able to edit it, you need to go to the brush dialog (Windows > Dockable dialogs > Brush), then here you can duplicate it then edit your duplicate.
Patrice