Amending Rows based on Duplicate Count

Hi muggydaniel ,

In PQ you can  use Table. Group and then from the resulting group, add an index column. Those with 0 index is the main row else the duplicate. Here's a sample code:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WckxJzFWK1cHGcM4oyixGYznmlWTk51UqxcYCAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Customer = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Customer", type text}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Customer"}, {{"Group", each _, type table [Customer=nullable text]}}),
    #"Added Custom" = Table.AddColumn(#"Grouped Rows", "Custom", each Table.AddIndexColumn([Group], "Index" ), type table),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"Group"}),
    #"Expanded Custom" = Table.ExpandTableColumn(#"Removed Columns", "Custom", {"Index"}, {"Index"}),
    #"Added Custom1" = Table.AddColumn(#"Expanded Custom", "Main/Duplicate", each if [Index] = 0 then "Main" else "Duplicate", type text)
in
    #"Added Custom1"

For small tables, the approach above is fine but on a very large table I would prefer DAX as it is more optimized at scanning a very large table than PQ. After adding an index column and loading the table, I would create this calculated column (not a measure)

Main/Duplicate = 
IF (
    CALCULATE ( MIN ( 'DAX'[Index] ), ALLEXCEPT ( 'DAX', 'DAX'[Customer] ) ) = 'DAX'[Index],
    "Main",
    "Duplicate"
)

Please see attached pbix for your reference.