Public Function ContainsUniqueEntries _
     (ByVal dt As DataTable, ByVal Columnindex As Integer) As Boolean
        Dim enm As IEnumerable(Of Object) = _
        From row As DataRow In dt Select row.Item(Columnindex) Distinct
        Dim cc As Integer = Aggregate el As Object In enm Into Count()
        Return cc = dt.Rows.Count
    End Function
 
 
    Private Sub DataGridView1_CellEndEdit(ByVal sender As Object, _
        ByVal e As System.Windows.Forms.DataGridViewCellEventArgs) _
        Handles DataGridView1.CellEndEdit
 
        If Not ContainsUniqueEntries(tab_DNA, e.ColumnIndex) Then
            MsgBox("Doublette in der Spalte " & cstr(e.columnindex))
        End If
 
    End Sub