NonUniqueObjectException on saving joined tables

Question

I have two related tables in a many to many relation: Role and Permission. The join table is RolePermission.

The Role object has a list of permission. it looks like this:

private Set<Permission> Permissions;    

@ManyToMany(fetch = FetchType.EAGER, cascade = { CascadeType.ALL })
@JoinTable(name = "permission_role", joinColumns = @JoinColumn(name = "role_id"),
           inverseJoinColumns = @JoinColumn(name = "permission_id"))
public Set<ObjectPermission> getObjectPermissions() {
    return objectPermissions;
}

In a certain time I want to add permissions to a role:

role.getObjectPermissions().add(permission);
roleDAO.saveOrUpdate(role);

Both the role and the permission exist already in the db, each in its table. I want to add a record to the relation table, that will point both on the role and the permission:

role.addObjectEntryPermissionRole(permission);
roleDAO.saveOrUpdate(role);

The problem is that it looks like it's trying to save the permission itself! Then it throws this error:

org.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session: [com.XXX.model.Permission#11]

Could somebody tell me what's the problen? what did I do wrong?