create a sub label

I am trying to create a field of sub-labels. I have a column labeled with numbers, but many repeat. See below:

1
1
2
2
2
3
4
4
4
4
5
etc......

And I am trying to create a column where I can uniquely identify these numbers. See below:
1A
1B
2A
2B
2C
3A
4A
4B
4C
4D
5A
etc.....

Does anyone know a Macro or anyway I can create these letters inside of a column, relating to the numbers?

Thanks a lot!