The NetBeans GUI designer doesn't support this directly, but you can instantiate your own custom components as often as desired. The key is designing for reuse. I try to follow the Swing separable model architecture, even if there's no need for a custom look & feel.
Edit: I added two instances of NewJPanel
to Main
like in this example. When I changed the label's text in the designer, it changed in both panels at runtime.
f.setLayout(new GridLayout(0, 1));
f.add(new NewJPanel());
f.add(new NewJPanel());