Countif to find 2nd occurence in range

I have a range that includes entries with 2 A's, 2 B's, 2 C's etc.
I want to create a duplicate range with the 1st A as A-1, the 2nd A as A-2 and 1st B as B-1, the 2nd B as B-2, etc. (the attached file shows these in cells)

I have tried
=IF(B1="","",B1&"-"&COUNTIF($B$1:$B1,B1))
but the results are inconsistent, see 3rd range values in red

The attached file has 3 ranges, from top to bottom:
1) starting data
2) how I want the results to look
3) my approach with ....countif mentioned above.

Can someone show me how to modify the eqn to give me correct results in all the cells?
thx