Вы можете разделить, удалить дубликаты и сгруппировать снова.
let
Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("Hck7CoBADIThu6SeRj1O2CKuAcO+IKjg7V3TDD/zMdNCoKKPdbyjC1zyqRXxUALTOn33Oyua1aqOYi1gmyCHNLhdgqh/KKUP", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [Column1 = _t, Column2 = _t]),
#"Changed Type" = Table.TransformColumnTypes(Source,{{"Column1", Int64.Type}, {"Column2", type text}}),
#"Split Column by Delimiter" = Table.ExpandListColumn(Table.TransformColumns(#"Changed Type", {{"Column2", Splitter.SplitTextByDelimiter(",", QuoteStyle.None), let itemType = (type nullable text) meta [Serialized.Text = true] in type {itemType}}}), "Column2"),
#"Changed Type1" = Table.TransformColumnTypes(#"Split Column by Delimiter",{{"Column2", type text}}),
#"Removed Duplicates" = Table.Distinct(#"Changed Type1"),
#"Grouped Rows" = Table.Group(#"Removed Duplicates", {"Column1"}, {{"Rows", each _, type table [Column1=number, Column2=text]}}),
#"Added Custom" = Table.AddColumn(#"Grouped Rows", "Custom", each [Rows][Column2]),
#"Extracted Values" = Table.TransformColumns(#"Added Custom", {"Custom", each Text.Combine(List.Transform(_, Text.From), ","), type text}),
#"Removed Columns" = Table.RemoveColumns(#"Extracted Values",{"Rows"})
in
#"Removed Columns"