Return a one for the first instance of duplicate values and a “zero” for the other instances

• Add an Index column to be able to sort back to original order
• Group by Customer ID
• Within each subgroup:
  • Add another index column to mark the first duplicate
  • Transform that Index column so the 2nd row (first duplicate) is a 1 and other entries are 0's

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WSlSK1YlWSgKTyWAyEYlMwpCFkClIZCoSmQYm05ViYwE=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Column1 = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Column1", type text}}),

//add index column to be able to sort back to original order
    #"Added Index" = Table.AddIndexColumn(#"Changed Type", "Index", 0, 1, Int64.Type),

//Group by the ID column (Column1 here)
//then add a column to each subgroup which has a one for the first duplicate entry (2nd entry) in each sub-table
    #"Grouped Rows" = Table.Group(#"Added Index", {"Column1"}, {
        {"First Duplicate", (t)=>
            Table.TransformColumns(
                Table.AddIndexColumn(t,"First Duplicate",0,1,Int64.Type),
                {"First Duplicate", each if _ = 1 then 1 else 0})}
            }),

//re-expand and sort back to original order
    #"Expanded First Duplicate" = Table.ExpandTableColumn(#"Grouped Rows", "First Duplicate", {"Index", "First Duplicate"}),
    #"Sorted Rows" = Table.Sort(#"Expanded First Duplicate",{{"Index", Order.Ascending}}),
    #"Removed Columns" = Table.RemoveColumns(#"Sorted Rows",{"Index"})
in
    #"Removed Columns"

If you want to mark the very first entry as a 1 (if there are duplicates), you can change that line in the Table.Group aggregation to mark the first entry if Table.RowCount is > 1