Public Class Form2
    Inherits System.Windows.Forms.Form
#Region " Vom Windows Form Designer generierter Code "
' ...
#End Region
    Public Shared IsOpen As Boolean = False
    Private Sub Form2_Closed(ByVal sender As Object, ByVal e As _
      System.EventArgs) Handles MyBase.Closed
        Me.IsOpen = False
    End Sub
    Private Sub Form2_Load(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles MyBase.Load
        IsOpen = True
    End Sub
End Class

        Dim f As Form2
        If Not Form2.IsOpen Then
            f = New Form2
            f.Show()
        Else
            MessageBox.Show("Die Form ist schon geöffnet!")
        End If