How do i avoid opening duplicate MDI child form which is already open. I am using the following codes for opening MDI child form:

Code:
Private Sub EmployeeToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles EmployeeToolStripMenuItem.Click
        Dim childform As New Employee
        childform.MdiParent = Me
        childform.Show()
    End Sub
What changes should be made ?