Repeated Index based on 2 grouped by columns

Here's how to do it in PQ without using any intermediate table and any manual editing of the M code. Everything done by just point-and-click:

// T
let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45W8gxR0lEy1jcy0TcyMDIEsg2VYnWoKWyBJGxMUBiHIWZIwkbUE/YIAjLN9Y2MUK2kubAFkrARQWFLJGFjgsKGBkjiJqSLxwIA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Category = _t, Date = _t, OriginalIndex = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Category", type text}, {"Date", type text}, {"OriginalIndex", Int64.Type}}),
    #"Changed Type with Locale" = Table.TransformColumnTypes(#"Changed Type", {{"Date", type date}}, "en-US"),
    #"Sorted Rows" = Table.Sort(#"Changed Type with Locale",{{"Category", Order.Ascending}, {"Date", Order.Ascending}}),
    #"Grouped Rows" = Table.Group(#"Sorted Rows", {"Category", "Date"}, {{"RowsToExpandTo", each _, type table [Category=nullable text, Date=nullable date, OriginalIndex=nullable number]}}),
    #"Added Index" = Table.AddIndexColumn(#"Grouped Rows", "Index", 1, 1, Int64.Type),
    #"Grouped Rows1" = Table.Group(#"Added Index", {"Category"}, {{"MinIndex", each List.Min([Index]), type number}, {"RowsToExpand2", each _, type table [Category=nullable text, Date=nullable date, RowsToExpandTo=table, Index=number]}}),
    #"Reordered Columns" = Table.ReorderColumns(#"Grouped Rows1",{"Category", "RowsToExpand2", "MinIndex"}),
    #"Expanded RowsToExpand2" = Table.ExpandTableColumn(#"Reordered Columns", "RowsToExpand2", {"Category", "Date", "RowsToExpandTo", "Index"}, {"Category.1", "Date", "RowsToExpandTo", "Index"}),
    #"Removed Columns" = Table.RemoveColumns(#"Expanded RowsToExpand2",{"Category.1"}),
    #"Added Custom" = Table.AddColumn(#"Removed Columns", "FinalIndex", each [Index] - [MinIndex] + 1),
    #"Removed Columns1" = Table.RemoveColumns(#"Added Custom",{"Index", "MinIndex", "Category", "Date"}),
    #"Expanded RowsToExpandTo" = Table.ExpandTableColumn(#"Removed Columns1", "RowsToExpandTo", {"Category", "Date", "OriginalIndex"}, {"Category", "Date", "OriginalIndex"}),
    #"Added Custom1" = Table.AddColumn(#"Expanded RowsToExpandTo", "Is Final Equal to Original?", each [FinalIndex] = [OriginalIndex])
in
    #"Added Custom1"

Just inspect the code one step at a time and you'll be able to apply this technique to anything. Enjoy!