file uploading in .net c#

You simply need to maintain the state of the upload. If and once you upload a file, keep a session flag like Session["IsUploaded"] = true;

Now, before checking for the upload, all you need is to see the value of this Session field first. If you find it as set true, then pass on the verification/validation check and move on to next page.

UPDATE:
I re-read the question and it sounded like you have the validation check on client side. (since you said it is the case of browser back button). Browser back button works on browsers cache and the page is not reloaded until asked. Either reload the page or keep the flag tracking the upload on client side and update it once uploaded. Try out which one works for you.