Column A - Column B - Column C
123 --------A1
123---------A1-------------I
123---------B1
567---------A1
567---------B1
567---------C1
567---------A1-------------I

If, per Number in Column A, in Column B are DUPLICATION, then assign in Column C, "I" beginning second appearance
For 123, A1 appear 2 times
For 567, A1 appear 2 times