Primary key constraint violated using (Oracle) sequence to generate ID in multithreaded app

Here's how I annotate the ID:
@Id
@SequenceGenerator(name = "FooSeq", sequenceName = "seq_foo", allocationSize = 20)
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "FooSeq")
private Long id;

Here's how I create the (Oracle) sequence:
CREATE SEQUENCE seq_foo START WITH 1 INCREMENT BY 1;

I get a primary key unique constraint violated in a multithreaded app i.e. it doesn't happen in single-threaded!

You can simply reproduce this error by either create blocking queue or blocking thread pool say size 5 to insert 10000+ object.