I have two separate forms, one is invoice, one is ledger

My client wants to join those forms into one, when he finishes entering the invoice, will auto-run the ledger form.

On the invoice form, I added a command button, when click, it will run the ledger form, which works.

But my client doesn't want to click the button.

so on the invoice.save, I added:
do form ledger

When I save the invoice, it will keep running the ledger form over and over.
How can I make it only runs once?

Thanks for the help

Jerry Yang