Replace second or more instances of duplicates with null - excel

Question

I have the following data with duplicates which I wish to identify. I do not wish to remove these so unique value only won't work. I want to be able to identify them but just saying null.
I have attempted to self-reference the code but end up just duplicating the original result.
let
Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
#"Changed Type" = Table.TransformColumnTypes(Source,{{"Column1", type text}}),
#"Removed Duplicates" = Table.Distinct(#"Changed Type"),
#"Merged Queries" = Table.NestedJoin(Source, {"Column1"}, #"Removed Duplicates", {"Column1"}, "Removed Duplicates", JoinKind.LeftOuter)
in
#"Merged Queries"

Answer 1

You can use List.Generate to generate a list with your requirements. And then you can either replace the first column or add the list as a second column.
This needs to be done in the Advanced Editor.
Please note there is a difference between the text string "null" and the power query null value. Based on your example screenshot, I assumed you wanted the "null" text string. If you prefer the null value, remove the surrounding quotes in the code
M Code
let
//Change next line to reflect your actual data source
Source = Excel.CurrentWorkbook(){[Name="Table13"]}[Content],
#"Changed Type" = Table.TransformColumnTypes(Source,{{"Column1", type text}}),
//change 2nd and later duplicates to null
dupsNull = List.Generate(
()=>[v=#"Changed Type"[Column1]{0}, idx=0],
each [idx]<Table.RowCount(#"Changed Type"),
each [v=if List.PositionOf(#"Changed Type"[Column1],#"Changed Type"[Column1]{[idx]+1},Occurrence.First) = [idx]+1
then #"Changed Type"[Column1]{[idx]+1} else "null", idx=[idx]+1],
each [v]),
//either add as a column or replace the first column
#"add de-duped" = Table.FromColumns(
Table.ToColumns(#"Changed Type") & {dupsNull},
type table[Column1=text, Column2=text])
in
#"add de-duped"

Answer 2

Here's another way. First, add an index column. Then add another column using List.PositionOf to get the row of the first occurrence of each value in the column. Then add one last column to compare the index and List.PositionOf, to determine which row entries should be null.
Let
Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
#"Changed Type" = Table.TransformColumnTypes(Source,{{"Column1", type text}}),
#"Added Index" = Table.AddIndexColumn(#"Changed Type", "Index", 0, 1, Int64.Type),
#"Added Custom" = Table.AddColumn(#"Added Index", "Custom", each List.PositionOf(#"Added Index"[Column1],[Column1])),
#"Added Custom1" = Table.AddColumn(#"Added Custom", "Custom.1", each if [Index] = [Custom] then [Column1] else null)
in
#"Added Custom1"

Answer 3

Here a solution that doesn't require to add a new column. It returns the same column just with duplicated values replaced with "null":
let
Source = Excel.CurrentWorkbook(){[Name="TB_INPUT"]}[Content],
#"Changed Type" = Table.TransformColumnTypes(Source,{{"Column1", type text}}),
removeDups = (lst as list) =>
List.Accumulate(lst, {}, (x, y) => x & {if List.Contains(x, y) then "null" else y}),
replacedValues = removeDups(Table.Column(#"Changed Type", "Column1")),
#"replaced Values" = Table.FromList(replacedValues, null, type table[Column1 = Text.Type ])
in
#"replaced Values"
it uses a List.Accumulate function to simplify the process to generate the corresponding list with the specified requirements.
The output will be the following in Power Query:
and in Excel:
If you want an empty cell instead of "null" token, then in the function removeDups replace "null" with null.
If you want to consider a more general case, lets say you have more than one column in the input Excel Table and you want to replace duplicated values in more than one column at the same time.
Let's say we have the following input in Excel:
The following code can be used to replace duplicates in Column1 and Column2:
let
Source = Excel.CurrentWorkbook(){[Name="TB_GralCase"]}[Content],
#"Changed Type" = Table.TransformColumnTypes(Source,{{"Column1", type text}, {"Column2", Int64.Type}}),
listOfColumns = {"Column1", "Column2"},
remainingColumns = List.Difference(Table.ColumnNames(#"Changed Type"), listOfColumns),
removeDups = (lst as list) =>
List.Accumulate(lst, {}, (x, y) => x & {if List.Contains(x, y) then "null" else y}),
replacedValues = List.Transform(listOfColumns, each removeDups(Table.Column( #"Changed Type", _))),
#"replaced values" = Table.FromColumns(
replacedValues & Table.ToColumns(Table.SelectColumns( #"Changed Type", remainingColumns)),
listOfColumns & remainingColumns
)
in
#"replaced values"
In listOfColumns variable, you define the list of columns you want to replace duplicates.
The the output in Power Query will be: