!Page.IsPostBack

I have dropdown list in Login page. The data is adding twice because the Page_Load event is executing twice. But I am checking !Page.IsPostBack property. First time the value for '!Page.IsPostBack' is 'true'. When it goes for second time it is not setting to 'false'. Please remember the '!' before the property. Which is means that still it is not recognizing that the page is loaded. Basically the Login page is redirecting from the Authenticate page. Any help please, thanks in advance.