Issue with same names.

Hi, Anonymous ;

In power query ,you could add new column then delete original column.

See the working here - Open a blank query - Home - Advanced Editor - Remove everything from there and paste the below code to test.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WclTSUTIEYielWB0IzwiInZViYwE=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Employee = _t, ID = _t, Manager = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Employee", type text}, {"ID", Int64.Type}, {"Manager", type text}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Employee"}, {{"Count", each Table.RowCount(_), Int64.Type}, {"a", each _, type table [Employee=nullable text, ID=nullable number, Manager=nullable text]}}),
    #"Expanded a" = Table.ExpandTableColumn(#"Grouped Rows", "a", {"ID", "Manager"}, {"ID", "Manager"}),
    #"Added Conditional Column" = Table.AddColumn(#"Expanded a", "Custom", each if [Count] = 1 then [Employee]
 else if [Manager]="B" then
      [Employee]&"_1"
    else [Employee]&"_2"),
    #"Removed Columns" = Table.RemoveColumns(#"Added Conditional Column",{"Employee", "Count"})
in
    #"Removed Columns"

The final show:

Best Regards,
Community Support Team _ Yalan Wu
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.