delete duplicate in a column without deleting the rows

When tested using the Power BI tools, this code seems to execute much more rapidly than yours.

• Group by Product ID
• Add a Shifted stock column to the sub-table
• If the contents of Stock and Shifted Stock are the same, then replace Stock with a null
• Remove the Shifted column
• Re-expand the table

let
    Source = Excel.CurrentWorkbook(){[Name="Table9"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Customer ID", type text}, {"Product ID", Int64.Type}, {"Sales", Int64.Type}, {"Stock", Int64.Type}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Product ID"}, {
        {"all", (t)=> 
            let 
                #"Add Shifted" = Table.FromColumns( 
                        Table.ToColumns(t) & 
                        {{null} & List.RemoveLastN(t[Stock],1)},
                        Table.ColumnNames(t) & {"Shifted"}),
                #"Null Dup Stock" = Table.ReplaceValue(
                    #"Add Shifted",
                    each [Stock],
                    each [Shifted],
                    (x,y,z)=> if y = z then null else y,
                    {"Stock"}),
                #"Remove Shifted" = Table.RemoveColumns(#"Null Dup Stock","Shifted")
            in 
                #"Remove Shifted",
            type table[Customer ID=text, Product ID=Int64.Type, Sales=Int64.Type, Stock=Int64.Type]}
        
        
        }),
    #"Removed Columns" = Table.RemoveColumns(#"Grouped Rows",{"Product ID"}),
    #"Expanded all" = Table.ExpandTableColumn(#"Removed Columns", "all", Table.ColumnNames(#"Changed Type"))
in
    #"Expanded all"

Results from your data

Hi Sofinobi, different approach here.

Result

let
    Source = Excel.Workbook(File.Contents("C:\Downloads\2024 Test duplicate.xlsx"), null, true),
    Sales_Table = Source{[Item="Sales",Kind="Table"]}[Data],
    GroupedRows = Table.Group(Sales_Table, {"Product ID"}, {{"All", each 
        [ a = Table.RemoveColumns(_, {"Stock"}),
          b = Table.FromColumns(Table.ToColumns(a) & {{[Stock]{0}?} & List.Repeat({null}, Table.RowCount(a)-1)}, Value.Type(_))
        ][b], type table}}),
    CombinedAll = Table.Combine(GroupedRows[All])
in
    CombinedAll