Private Sub Button1_Click(ByVal sender As System.Object, _
  ByVal e As System.EventArgs) Handles Button1.Click
 
  With OpenFileDialog1
    If .ShowDialog() = Windows.Forms.DialogResult.OK Then
      ' aktive ListBox finden
      For Each oControl As ListBox In TabControl1.SelectedTab.Controls
        oControl.Items.AddRange(.FileNames)
        Exit For
      Next
    End If
  End With
End Sub

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