An IN, ANY, or EXISTS subquery evaluates to true if there is at least one row that causes the subquery to evaluate to true. These semantics make a DISTINCT within an IN, ANY, or EXISTS subquery unnecessary. The following two queries are equivalent and the first is transformed into the second:

SELECT * FROM t1 WHERE c1 IN
    (SELECT DISTINCT c2 FROM t2 WHERE t1.c3 = t2.c4)

SELECT * FROM t1 WHERE c1 IN
    (SELECT c2 FROM t2 WHERE t1.c3 = t2.c4)