assign the same serial no, for duplicate transaction rows

mark_dups.vba

Sub MarkDuplicates()
    ' Activate the Master Sheet
    Sheets("master").Activate
    
    ' Count the number of non-empty rows in the master sheet
    RowCount = Worksheets("master").Cells(Rows.Count, 1).End(xlUp).Row
    ColCount = Worksheets("master").Cells(1, Columns.Count).End(xlToLeft).Column

    dealAmtCol = 3
    firstRow = 1
    lastRow = RowCount
    lastCol = ColCount
    idx = 1
    

    For J = firstRow To lastRow
        If Range(Cells(J, 1), Cells(J, lastCol)).Interior.Color <> 255 Then
            For i = J + 1 To lastRow + 1
                If Cells(i, dealAmtCol) = Cells(J, dealAmtCol) Then
                    
                    If Range(Cells(J, 1), Cells(J, lastCol)).Interior.Color <> 255 Then
                        Cells(J, 1).Value = idx
                        Range(Cells(J, 1), Cells(J, lastCol)).Interior.Color = 255
                        
                        Cells(i, 1).Value = idx
                        Range(Cells(i, 1), Cells(i, lastCol)).Interior.Color = 255
                        
                    Else
                        Cells(i, 1).Value = Cells(J, 1).Value
                        Range(Cells(i, 1), Cells(i, lastCol)).Interior.Color = 255
                    End If
                Else
                    If Range(Cells(J, 1), Cells(J, lastCol)).Interior.Color <> 255 Then
                        Cells(J, 1).Value = idx
                        Range(Cells(J, 1), Cells(J, lastCol)).Interior.Color = 255
                    End If
                End If
            Next
        idx = idx + 1
        End If
    Next
    
    For Each cell In Range(Cells(firstRow, 1), Cells(RowCount, ColCount))
      cell.Interior.Color = xlNone
    Next
    
End Sub