Stop form re-submission on back ?

[eluser]junaids[/eluser]
can u explain this

Code:

$this->output->set_header('Cache: No-cache');

and where should this line be placed in code?

plus i have a different question...
when a user is logs in( submits username, password) to application...he is directed to his homepage. if he refreshes the page...username password is resubmitted and again authenticated..how can i stop this?