Private WithEvents frm As Form2
 
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        If frm Is Nothing Then
            frm = New Form2
            frm.Text = "kommt von Button 1"
            frm.Show()
        End If
End Sub
 
Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button2.Click
        If frm Is Nothing Then
            frm = New Form2
            frm.Text = "kommt von Button 2"
            frm.Show()
        End If
End Sub
 
Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button3.Click
        If frm Is Nothing Then
            frm = New Form2
            frm.Text = "kommt von Button 3"
            frm.Show()
        End If
End Sub