How to query results where a column with 2 classifications has same value in another column in the same table? I have a table like this:

Column A    Column B
-------------------------
Agent A     abc@gmail.com
Agent B     abc@gmail.com
Agent A     abc@gmail.com
Agent B     def@gmail.com 
Agent C     efg@gmail.com


I want to see results where the same email address exists for Agents A and B.
Something like this:

select a.columna, a.email
from (select * from tablename where columna='A') a
join (select * from tablename where columna='B') b
on (a.columnB=b.ColumnB)