COUNTIF or AGGREGATE Merge in Power Query

If you absolutely, for sure, no question need this as a calculated DAX column, there are more efficient ways than using MAXX and COUNTX.

ReturnCount = COUNTROWS( FILTER( WorkflowData, WorkflowData[Non-Unique Key] = [Unqiue Key]))

You can also do this in power query, if you don't want to load in all those rows in the first place.  The lines in red are where the joining happens.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjE2NtZ1LC0uKcpMVIrViVYyN7Yw0XVPLcpNzKtUio0FAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [Key = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Key", type text}}),
    #"Renamed Columns" = Table.RenameColumns(#"Changed Type",{{"Key", "Unique Key"}}),

    #"CountedKeys" = Table.Group(WorkflowData, {"Non-Unique Key"}, {{"ReturnCount", each Table.RowCount(_), type number}}),
    #"Merge Tables" = Table.Join(#"Renamed Columns", "Unique Key", #"CountedKeys", "Non-Unique Key")
in
    #"Merge Tables"