Dim duplicates = DataSourceTable.AsEnumerable().GroupBy(Function(i) i.Field(Of String)("FieldName")).Where(Function(g) g.Count() > 1).Select(Function(g) g.Key)
    DataSourceTable = New DataTable
    Dim row As DataRow
    For Each dup In duplicates
        row = DataSourceTable.NewRow
        'row(0) = dup// I want get all values columns with index
    Next

Dim duplicates = DataSourceTable.AsEnumerable()
                                .GroupBy(Function(i) i.Field(Of String)("FieldName"))
                                .Where(Function(g) g.Count() > 1)
                                .Select(Function(g) g.Key)

Dim duplicates = DataSourceTable.AsEnumerable()
                                .GroupBy(Function(i) i.Field(Of String)("FieldName"))
                                .Where(Function(g) g.Count() > 1)

Dim duplicates = DataSourceTable.AsEnumerable()
                            .GroupBy(Function(i) i.Field(Of String)("FieldName"))
                            .Where(Function(g) g.Count() > 1)