How To Count The Number Of Unique Values In A List, Based On Another Value

I need to count the unique values in a list based on another value. i.e.

AB ___ Jones
AB ___ Smith
BC ___ Henry
AB ___ Jones
BC ___ Brown
BC ___ Henry
AB ___ Tomkins

For all the AB's in column 1, I need a formula that will give me all the unique names from column 2, in this case 3. I've tried adapting:

=IF(LEN(June!D5:D100),SUMPRODUCT(1/COUNTIF(June!D5:D100,June!D5:D100)))

but haven't been able to get it to work.