Message: Sys.InvalidOperationException: Type SP.UI.DialogResult has already been registered. The type may be defined multiple times or the script file that defines it may have already been loaded.
My workflow: I tried to open a dialog by clicking a button so I used SP.UI.ModalDialog
However when the page loaded, the error occurred.
<asp:Content ID="PageHead" ContentPlaceHolderID="PlaceHolderAdditionalPageHead" runat="server">
<link href="../Stylesheet.css" rel="stylesheet" type="text/css" />
I am using SharePoint 2010, Visual Studio 2010.
Thank you very much.