Found shared references to a collection

Assuntos Gerais
#1

Olá, estou tendo a seguinte exception Found shared references to a collection e não estou conseguindo visualizar o problema no relacionamento, preciso persistir uma lista de usuários que receberão um email, abaixo segue os modelos:

Model User:

@javax.persistence.Entity
@Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
public class User extends Entity {
		
	@Id 
	@GeneratedValue(strategy=GenerationType.AUTO, generator="h_seq_user")
	@SequenceGenerator(name="h_seq_user", sequenceName="h_seq_user")
	@Index(name="index_001_user")
	private Long id;
	

	@NotNull(message="#{default.NotNull}")
	@Column(length=100,unique=true)
	@Size(min=10,max=100,message="{user.NameSize}")
	@Index(name="index_002_user")
	private String name;
	
	
	@NotNull(message="#{default.NotNull}")
	@Column(length=15,unique=true)
	@Size(min=5,max=15,message="{user.UserNameSize}")
	@Index(name="index_003_user")
	private String username;
	

	@Column(name = "picture")
	private byte[] picture;
	

	private boolean active;
	
	
	
	@NotNull(message="#{default.NotNull}")
    @Pattern(regexp = "^[\\w\\-]+(\\.[\\w\\-]+)*@([A-Za-z0-9-]+\\.)+[A-Za-z]{2,4}$", message="{user.Email}")
	@Column(length=250,unique=true)
	private String email;
	


	@ManyToMany(
	        cascade = {CascadeType.REFRESH},
	        mappedBy = "users",
	        targetEntity = Groupuser.class
	    )
	private List <Groupuser> groupusers;	

	
	@ManyToMany(
	        cascade = {CascadeType.REFRESH},
	        mappedBy = "users",
	        targetEntity = Email.class
	    )
	private List <Email> emails;

//Get´s e Set´s

Model Email:

@javax.persistence.Entity
@Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
public class Email extends Entity {

	
	@Id 
	@GeneratedValue(strategy=GenerationType.AUTO, generator="h_seq_email")
	@SequenceGenerator(name="h_seq_email", sequenceName="h_seq_email")
	@Column(nullable=false)
	@Index(name="index_001_email")
	private Long id;	

	
	
	@NotEmpty(message="{default.NotNull}")
	@Column(length=120,nullable=false)
	private String emailfrom;
	

	@NotEmpty(message="{default.NotNull}")
	@Column(length=300,nullable=false)
	private String subject;


	@NotEmpty(message="{default.NotNull}")
	@Column(length=5000,nullable=false)
	private String msg;
	

	@NotNull(message="{default.NotNull}!")
	@Index(name="index_002_email")
	@ManyToOne
	@JoinColumn(name="user_id")
	private User user;
	
	
	
	@NotNull(message="{default.NotNull}")
	@ManyToMany(
	        targetEntity=User.class,
	        cascade={CascadeType.REFRESH}
	    )
	    @JoinTable(
	        name="email_user",
	        joinColumns=@JoinColumn(name="email_id"),
	        inverseJoinColumns=@JoinColumn(name="user_id")
	    )	
	private List <User> users;

Método onde estou tendo problema:

	public void sendEmailUpdateUser(User user){
		
		List<User> users = groupuser_repository.find(4l).getUsers();
		
		Email email = new Email();
		email.setMsg("Data updated");
		email.setSubject("User updated");
		email.setUser(session_user.getLogged());
		email.setUsers(users);
		email.setEmailfrom("my.email@provedor.com.br");
		
	   
		validator.validate(email);
		validator.onErrorUsePageOf(UserController.class).edit(user);
		repository.create(email);
		
		
		result.redirectTo(UserController.class).index();
		
	}

Exception:

Caused by: org.hibernate.HibernateException: Found shared references to a collection: br.com.app.models.Email.users
	at org.hibernate.engine.internal.Collections.processReachableCollection(Collections.java:178)
	at org.hibernate.event.internal.FlushVisitor.processCollection(FlushVisitor.java:59)
	at org.hibernate.event.internal.AbstractVisitor.processValue(AbstractVisitor.java:121)
	at org.hibernate.event.internal.AbstractVisitor.processValue(AbstractVisitor.java:82)
	at org.hibernate.event.internal.AbstractVisitor.processEntityPropertyValues(AbstractVisitor.java:76)
	at org.hibernate.event.internal.DefaultFlushEntityEventListener.onFlushEntity(DefaultFlushEntityEventListener.java:174)
	at org.hibernate.event.internal.AbstractFlushingEventListener.flushEntities(AbstractFlushingEventListener.java:225)
	at org.hibernate.event.internal.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:99)
	at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:51)
	at org.hibernate.internal.SessionImpl.flush(SessionImpl.java:1210)
	at org.hibernate.internal.SessionImpl.managedFlush(SessionImpl.java:399)
	at org.hibernate.engine.transaction.internal.jdbc.JdbcTransaction.beforeTransactionCommit(JdbcTransaction.java:101)
	at org.hibernate.engine.transaction.spi.AbstractTransactionImpl.commit(AbstractTransactionImpl.java:175)
	at org.hibernate.ejb.TransactionImpl.commit(TransactionImpl.java:75)
	... 43 more

Alguém tem uma idéia?

#2