Dynamic Columns duplicating

Hi GoatTracker ,
then you have to use a complete different approach, as you started with a cartesian product already:

let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Task", type text}, {"Start", type date}, {"End", type date}}),
    #"Merged Queries" = Table.NestedJoin(#"Changed Type", {"Start"}, #"Date Dimension", {"FullDateAlternateKey"}, "Date Dimension", JoinKind.LeftOuter),
    #"Added MonthStart" = Table.ExpandTableColumn(#"Merged Queries", "Date Dimension", {"MonthStart"}, {"MonthStart"}),
    #"Merged Queries1" = Table.NestedJoin(#"Added MonthStart", {"End"}, #"Date Dimension", {"FullDateAlternateKey"}, "Date Dimension", JoinKind.LeftOuter),
    #"Expanded Date Dimension" = Table.ExpandTableColumn(#"Merged Queries1", "Date Dimension", {"MonthStart"}, {"MonthEnd"}),
    #"Added Custom" = Table.AddColumn(#"Expanded Date Dimension", "Custom", each Record.Combine(List.Transform({1..20}, (l) => Record.AddField([], "Column" & Text.From(l), if [MonthStart]<=0 and [MonthEnd]>=0 then "x" else "y") ))),
    #"Expanded Custom" = Table.ExpandRecordColumn(#"Added Custom", "Custom", {"Column1", "Column2", "Column3", "Column4", "Column5", "Column6", "Column7", "Column8", "Column9", "Column10", "Column11", "Column12", "Column13", "Column14", "Column15", "Column16", "Column17", "Column18", "Column19", "Column20"}, {"Column1", "Column2", "Column3", "Column4", "Column5", "Column6", "Column7", "Column8", "Column9", "Column10", "Column11", "Column12", "Column13", "Column14", "Column15", "Column16", "Column17", "Column18", "Column19", "Column20"})
in
    #"Expanded Custom"