SharePoint Designer 2010 Javascript errors Sys.InvalidOperationException

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.

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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" />
    <script src="SP.UI.Dialog.js" type="text/javascript"></script>
</asp:Content>

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I am using SharePoint 2010, Visual Studio 2010.
Thank you very much.