Hibernate - mapping three tables with annotations

I have three tables: User, Role, Board. Each user can have many boards, but in one board user can have only one role. I created three classes and I added annotations:

class User {

   @Id
   @GeneratedValue
   @Column(name = "id")
   private long id;

   @Column(name = "mail")
   private String mail;
   ...

   @JoinTable(name = "userBoardRole",
        joinColumns = @JoinColumn(name = "id", unique = false),
        inverseJoinColumns = @JoinColumn(name = "role_id", unique = false))
   @MapKeyJoinColumn(name = "board_id", unique = false)
   @ElementCollection
   private Map<Board, Role> boardRoleMap = new HashMap<Board, Role>();
}

class Role {

   @Id
   @GeneratedValue
   @Column(name = "role_id")
   private long id;

   @Column(name = "name")
   private String name;
   ...
}

class Board {
   @Id
   @GeneratedValue
   @Column(name = "role_id")
   private long id;

   @Column(name = "name")
   private String name;
   ...
}

Hibernate create a new table, which look like this:

CREATE TABLE userboardrole ( id bigint(20) NOT NULL, role_id bigint(20) NOT NULL, board_id bigint(20) NOT NULL, PRIMARY KEY (id,board_id), UNIQUE KEY UK_3lunj2moakkbpehqwqkjcvlf4 (role_id), KEY FK_3lunj2moakkbpehqwqkjcvlf4 (role_id), KEY FK_ta3fwgh4sln85f6f3jjbte38u (board_id), KEY FK_lgd2b2mph9qoc1pe2h9r2wu8u (id), CONSTRAINT FK_lgd2b2mph9qoc1pe2h9r2wu8u FOREIGN KEY (id) REFERENCES users (id), CONSTRAINT FK_3lunj2moakkbpehqwqkjcvlf4 FOREIGN KEY (role_id) REFERENCES boardrole (role_id), CONSTRAINT FK_ta3fwgh4sln85f6f3jjbte38u FOREIGN KEY (board_id) REFERENCES boards (board_id) ) ENGINE=InnoDB DEFAULT CHARSET=utf8;

The problem is that the column role_id is unique. Is there any way, using annotations, to set column unique to false? Thank you for any help.

I'm using hibernate 4.2.2.