This happens because of cache.
1. clear the sessions
2. clear the cache such that browser has no history (this will make back/forward button in browser grayed out disabled.)
Code for clearing cache can be put up in code behind as follows:
Response.Cache.SetCacheability(HttpCacheability.NoCache);
Response.Cache.SetExpires(DateTime.UtcNow.AddHours(-1));
Response.Cache.SetNoStore();
You can add somethin similar in form aspx if you want to place it there:
<meta http-equiv="Cache-Control" content="no-cache">
<meta http-equiv="Pragma" content="no-cache">
<meta http-equiv="Expires" content="0"></meta></meta></meta>