I have a database that sort of is put together like this:

There is one column with a unique identifier, another column that groups the unique identifier and then I am trying to add a 3rd column that is a count of how many unique identifiers there are for a specific group

This is what I have:

A 1 ?
A 2 ?
B 3 ?
B 4 ?
B 5 ?
C 6 ?

This is what I want

A 1 1
A 2 2
B 3 1
B 4 2
B 5 3
C 6 1

Can anybody help to identify a query to do this?

Thanks! (I'm new to SQL)