I have two tables and need to join them
You could use GROUP BY, if you only need the basic information from first_table:

SELECT
    T1.id,
    T1.username,
    T1.firstname,
    T1.surname,
    T1.address
FROM first_table AS T1
JOIN second_table AS T2 ON (T2.user_id=T1.id)
JOIN first_table AS T3 ON (T3.id=T2.id AND T3.id&lt;&gt;T2.user_id)
WHERE T3.username='johnie' 
GROUP BY T1.id;


Try it in SQLFiddle.

You can also test it with T3.username='alice'.