Replace duplicated values with null in the same column per a given list of columns

Hi davidleal ,
utilizing on what you have already, I would suggest the following approach:

let
    Source = Table.FromRows(
        Json.Document(
            Binary.Decompress(
                Binary.FromText(
                    "i45W8kvMTTVU0lFyziypBNGGBkqxOpjCRghhI6gwiDbGLmyCKWwMpE2BwrEA",
                    BinaryEncoding.Base64
                ),
                Compression.Deflate
            )
        ),
        let
            _t = ((type nullable text) meta [Serialized.Text = true])
        in
            type table [Name = _t, City = _t, Amount = _t]
    ),
    #"Changed Type" = Table.TransformColumnTypes(
        Source,
        {{"Name", type text}, {"City", type text}, {"Amount", Int64.Type}}
    ),
    ListOfColumns = {"Name", "City"},
    RemainingColumns = List.Difference(Table.ColumnNames(#"Changed Type"), ListOfColumns),
    RemoveDups = (lst as list) =>
        List.Accumulate(lst, {}, (x, y) => x & {if List.Contains(x, y) then null else y}),
    replaceValues = List.Transform(ListOfColumns, each RemoveDups(Table.Column(#"Changed Type", _))),
    Custom1 = Table.FromColumns(
        replaceValues & Table.ToColumns(Table.SelectColumns(#"Changed Type", RemainingColumns)),
        ListOfColumns & RemainingColumns
    )
in
    Custom1