为什么我的VSTO Outlook加载项触发两次？

    Private Sub ThisAddIn_Startup() Handles Me.Startup
        explorer = Me.Application.ActiveExplorer()
        AddHandler Application.ItemContextMenuDisplay, AddressOf Application_ItemContextMenuDisplay
        AddHandler Application.Startup, AddressOf Application_CommandBarMenuDisplay
    End Sub

Sub Application_CommandBarMenuDisplay()

            Dim cBar As Office.CommandBar = explorer.CommandBars(\"Standard\")
            btnCommandBarMenu = CType(cBar.Controls.Add(Office.MsoControlType.msoControlButton, Type.Missing, Type.Missing, Type.Missing, True), Office.CommandBarButton)

            With btnCommandBarMenu
                .BeginGroup = True
                .Style = MsoButtonStyle.msoButtonIconAndCaption
                .Caption = \"File TNRP Email\"
                .Tag = \"File TNRP Email\"
                .Picture = IPictureDisp.FromImage(My.Resources.label16)
                .Mask = IPictureDisp.MaskFromImage(My.Resources.label16)
            End With

            AddHandler btnCommandBarMenu.Click, AddressOf btn_CommandBarMenuClick

    End Sub

Sub Application_ItemContextMenuDisplay(ByVal CommandBar As Microsoft.Office.Core.CommandBar, ByVal Selection As Microsoft.Office.Interop.Outlook.Selection)

            btnContextMenu = CommandBar.Controls.Add(Office.MsoControlType.msoControlButton, Type.Missing, Type.Missing, Type.Missing, True)

            With btnContextMenu
                .BeginGroup = True
                .Visible = True
                .Style = MsoButtonStyle.msoButtonIconAndCaption
                .Caption = \"File TNRP Email\"
                .Tag = \"File TNRP Email\"
                .Picture = IPictureDisp.FromImage(My.Resources.label16)
                .Mask = IPictureDisp.MaskFromImage(My.Resources.label16)
            End With

            AddHandler btnContextMenu.Click, AddressOf btn_ContextMenuClick

End Sub