Back button - do not disable

I have a form with a predisplayed line item number, in this case 1, to start off. There is a checkbox to add another line item, so if they check the box and fill out the form and submit, it brings up the form again with line item 2 (continuous increment by 1 if the box is checked). If they fill out the form again and do not check the add another line item box, the form submits and goes to another page with a popup message indicating a successful transaction.

Here is the problem. On the message page, if they hit the back button on the browser, it goes back to the form, with line item 2 (which was already submitted). If they submit again, line item 2 will be written to the table again, and there can only be one line item.

If I make the line item a unique key in the table, the application will just blowup. Can it display a nice error message about duplicate line items ?

My other option would be to use javascript to disable the back button, but everyone says that is not recommened.

If that is the case, what do I do to avoid this situation ?

The sequence of events/screens works properly, but as soon as they use the back button, that is where I run into problems.