Custom column of non duplicates

Hi nathan_kul ,

Please refer to the following steps. You can adjust the Source step to match your actual data source.

let
    Source = Table.FromRecords({[ID = 1], [ID = 2], [ID = 2], [ID = 3], [ID = 3], [ID = 3]}),
    #"Grouped Rows" = Table.Group(Source, {"ID"}, {{"Data", each Table.AddIndexColumn(_, "Index", 0, 1, Int64.Type)}}),
    #"Expanded Data" = Table.ExpandTableColumn(#"Grouped Rows", "Data", {"Index"}, {"Index"}),
    #"Added Custom" = Table.AddColumn(#"Expanded Data", "ID2", each if [Index]=0 then [ID] else null),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"Index"})
in
    #"Removed Columns"

Best Regards,
Gao
Community Support Team

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If I misunderstand your needs or you still have problems on it, please feel free to let us know. Thanks a lot!

How to get your questions answered quickly --  How to provide sample data in the Power BI Forum

Hi nathan_kul, different approach here:
I've used Anonymous sample data.

Output

let
    Source = Table.FromRecords({[ID = 1], [ID = 2], [ID = 2], [ID = 3], [ID = 3], [ID = 3]}),
    GroupedRows = Table.Group(Source, {"ID"}, {{"All", each Table.FromColumns(Table.ToColumns(_) & {{[ID]{0}?}}, Value.Type(_ & #table(type table[Unique ID=Int64.Type],{}))), type table}}),
    Combined = Table.Combine(GroupedRows[All])
in
    Combined

lovely question, use this formula to reach this result

let
    Source = Excel.CurrentWorkbook(){[Name="Table3"]}[Content],
    #"Added Index" = Table.AddIndexColumn(Source, "Index", 0, 1, Int64.Type),
    #"Added Custom" = Table.AddColumn(#"Added Index", "Custom", each if List.Contains(List.FirstN(Source[ID],[Index]),[ID]) then "" else [ID])
in
    #"Added Custom"