Counting duplicates in Excel

I have a list of postcodes that includes duplicates. I would like to find out how many instances of each postcode there are.
For example I would like this:

GL15

GL15

GL15

GL16

GL17

GL17

GL17


...to become this:

GL15 - 3

GL15 - 3

GL15 - 3

GL16 - 1

GL17 - 2

GL17 - 2


...or ideally this:

GL15 - 3

GL16 - 1

GL17 - 3

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) |
+-------+-------------------+

Hope this helps!

+--------+-------+--------+-------------------+
  |   A    |   B   |   C    |         D         |
  +--------+-------+--------+-------------------+
1 | ToSort |       | ToSort |                   |
  +--------+-------+--------+-------------------+
2 |  GL15  |       | GL15   | =COUNTIF(A:A, C2) |
  +--------+-------+--------+-------------------+
3 |  GL15  |       | GL16   | =COUNTIF(A:A, C3) |
  +--------+-------+--------+-------------------+
4 |  GL15  |       | GL17   | =COUNTIF(A:A, C4) |
  +--------+-------+--------+-------------------+
5 |  GL16  |       |        |                   |
  +--------+-------+--------+-------------------+
6 |  GL17  |       |        |                   |
  +--------+-------+--------+-------------------+
7 |  GL17  |       |        |                   |
  +--------+-------+--------+-------------------+