Say you are working with a fab house that offers a special on boards of a certain size. Seeed has a deal on 100mm x 100mm PCBs, and if your project is smaller, you can panelize the job by placing multiple copies of your circuit within that area. Even if you circuits fit, make sure the fab house allows for panelization; Seeed, for example, distinguishes between one repeated design and multiple designs on the same board; OSH doesn’t support V-cuts at all. In the image above, you can see that I left the mfr a note on the dimension layer to score the interior so that I can snap the design apart later.

When it comes to the steps in Eagle, the trick is to prevent the program from trying to connect all those duplicate wires together. Here’s a solution that I’ve edited for clarity (original here):

1. Start with a finished design.
2. Close the board view.
3. Switch to the schematic view, then select all, copy, and paste. You need a full copy for every instance you wish to repeat on the panel.
4. Save and close schematic. Ignore the back-annotation errors.
5. Open the board. Copy and paste your design, making sure you have the same number of copies as you made in the schematic.
6. Save the board, leave it open, and then open the schematic as well.
7. Run ERC. I didn’t have any of the duplicate nets that they warn about in the link above, so see the original posting for tips if you run into this problem.
8. If you want to verify that everything duped correctly, rip it all up and autoroute again. If everything worked correctly, each circuit will keep to itself.