How to remove duplicates between initial date and 21 days

Wait what, are you running this in Excel?

let
    Source = Excel.CurrentWorkbook(),
    Table61617_2 = Source{[Name="Table61617_2"]}[Content],
    #"Removed Other Columns" = Table.SelectColumns(Table61617_2,{"Participant ID", "Course ID", "Course Completion Date"}),
    #"Changed Type" = Table.Buffer(Table.TransformColumnTypes(#"Removed Other Columns",{{"Course Completion Date", type date}})),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Participant ID", "Course ID"}, {{"Rows", each _, type table [Course Completion Date=nullable date]}}),
    Check = (tbl)=> Table.AddColumn(tbl,"Flag",(k)=> Table.RowCount(Table.SelectRows(tbl,each [Course Completion Date]<k[Course Completion Date] and [Course Completion Date]>k[Course Completion Date]-#duration(21,0,0,0)))),
    #"Added Custom1" = Table.AddColumn(#"Grouped Rows", "FlagNew", each if Table.RowCount([Rows])=1 then Table.AddColumn([Rows],"Flag",each 0) else Check([Rows])),
    #"Removed Other Columns1" = Table.SelectColumns(#"Added Custom1",{"Participant ID", "Course ID", "FlagNew"}),
    #"Expanded FlagNew" = Table.ExpandTableColumn(#"Removed Other Columns1", "FlagNew", {"Course Completion Date", "Flag"}, {"Course Completion Date", "Flag"})
in
    #"Expanded FlagNew"