Here Mudassar Ahmed Khan has explained with an example, how to avoid the following message in browser when browser refresh button or CTRL + F5 is clicked after form is submitted i.e. PostBack in ASP.Net.
To display the website again the web browser needs to resend the information you’ve previously submitted. If you are making purchase, you should click Cancel to avoid a duplicate transaction. Otherwise click retry to display the webpage again.
The problem is that when browser refresh button or F5 or CTRL + F5 function keys are pressed, the last executed event is again executed.