I don't think there is a good solution...
While searching you do multiple postbacks, then you redirect to another page (product details) and once you hit back button, you are trying to navigate to previous action which, in fact, is postback. Now browser coe into a play and the confirmation you see is the way most browsers handle it.
To avoid this issue you might consider following:
- either switch to query string/redirection strategy during search
- open product details in a new tab as a workaround