I would to create two macros or vba code modules. That will check the records in a column and find duplicates. The second macro would find a duplicate and increase the sequence number by 1 in the duplicate row. I am including a sample spreadsheet below. Tab "A" I would to find just the duplicates. Tab "B" would find the duplicates and renumber the sequence number. The sequence number is in brackets. mytest_macros.xlsx
Sub numberdups()
Dim cel As Range
Dim n As Long
Dim i As Long
For i = Range("A2").End(xlDown).Row To 2 Step -1
Set cel = Range("A" & i)
n = WorksheetFunction.CountIf(Range("A1", cel.Offset(-1)), cel.Value)
cel.Value = Replace(cel.Value, "[0]", "[" & n & "]")
Next i
End Sub
For the first part how do you want to display the duplicates? Color them?