Counting duplicates in Excel

I don’t know if it’s entirely possible to do your ideal pattern. But I found a way to do your first way: CountIF

+-------+-------------------+
|   A   |         B         |
+-------+-------------------+
| GL15  | =COUNTIF(A:A, A1) |
+-------+-------------------+
| GL15  | =COUNTIF(A:A, A2) |
+-------+-------------------+
| GL15  | =COUNTIF(A:A, A3) |
+-------+-------------------+
| GL16  | =COUNTIF(A:A, A4) |
+-------+-------------------+
| GL17  | =COUNTIF(A:A, A5) |
+-------+-------------------+
| GL17  | =COUNTIF(A:A, A6) |
+-------+-------------------+