Conditional Fill Down

See the working here - Open a blank query - Home - Advanced Editor - Remove everything from there and paste the below code to test

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WUorVQSYc0fi4BVOUYmMB", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Data = _t]),
    #"Replaced Value" = Table.ReplaceValue(Source,"",null,Replacer.ReplaceValue,{"Data"}),
    #"Duplicated Column" = Table.DuplicateColumn(#"Replaced Value", "Data", "Data1"),
    #"Duplicated Column1" = Table.DuplicateColumn(#"Duplicated Column", "Data", "Data2"),
    #"Filled Down" = Table.FillDown(#"Duplicated Column1",{"Data1"}),
    #"Filled Up" = Table.FillUp(#"Filled Down",{"Data2"}),
    Custom1 = Table.ReplaceValue(#"Filled Up",each [Data],each if [Data1]=[Data2] then [Data1] else [Data],Replacer.ReplaceValue,{"Data"}),
    #"Removed Columns" = Table.RemoveColumns(Custom1,{"Data1", "Data2"})
in
    #"Removed Columns"