CIviContribute - Fatal error (session cleared side-effect) on back button after sucessful contribution

To recreate in Firefox - complete a PayPal Pro contribution (or might happen w/ any type) - then hit back button x 2 to get back to start page (Main.php). Then click Continue button again (as someone might do if trying to make a 2nd 'similar' contribution). You get a a fatal error (i think it's 'no id found...'). [In IE the error occurs immediately on hitting the back button from Thank You page according to Liam K.]

Desirable behavior would be to give nice status msg - "This contribution has already been submitted. Click here if you want to make another contribution." ("here" is linked to contrib start page w/ same id).