This is browser normal behavior. if the current page is a result of a postback (button clicked rather than a hyperlink), then to do a refresh the browser needs to post the data again (rerunning the page logic);
because so many sites failed to handle this on their purchase page, the browsers had to add this message to prevent double buys.
How to suppress this behavior
Browser displays that message only if page is result of submitting form with method POST. Which is how any buttom works in .NET you can get away with that if any action (i.e. button click) you will end with Response.Redirect somewere. Then it will me method “GET”.