Back button - do not disable

Question

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.

Answer 1

If I disable caching on the form page and they hit the back
button to go back to the form page, what line item value would be
displayed ?
Ideally, if it was 2 and they hit the back button, the next
value would be three.
Disabling cache, cookies, and sessions was suggested by
coworkers, but nobody could figure out how to get the next value in
there.