Combine rows based on unique id

Here is the Power Query "M" code version:

let
    Source = Csv.Document(File.Contents("C:\temp\powerbi\group.csv"),[Delimiter=",", Columns=2, Encoding=1252, QuoteStyle=QuoteStyle.None]),
    #"Promoted Headers" = Table.PromoteHeaders(Source),
    #"Changed Type" = Table.TransformColumnTypes(#"Promoted Headers",{{"id", type text}, {"branche", type text}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"id"}, {{"Branches", each _, type table}}),
    #"Added Custom" = Table.AddColumn(#"Grouped Rows", "Custom", each Text.Combine([Branches][branche],",")),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"Branches"})
in
    #"Removed Columns"

Basically, group the rows but keep all as the aggregation, return a table for "branche". Then, do a text combine on that table to get all of the values formatted as a single list of values with a comma separator.