javax.persistence.NonUniqueResultException: query did not return a unique result: 3

**hi big community! someone can help me**
 **here: Role entity**

        @Entity
        @Table(name="roles")
        public class Role implements Serializable {
        @Id @GeneratedValue(strategy = GenerationType.AUTO)
        private Integer id;  
        private String obser;
        public Role() {
            super();
        }

    public Role(Integer id, String obser) {
        super();
        this.id = id;
        this.obser = obser;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getObser() {
        return obser;
    }

    public void setObser(String obser) {
        this.obser = obser;
    }}



    @Entity
    @Table(name="users")
    public class User implements Serializable{

    @Id @GeneratedValue(strategy=GenerationType.AUTO)
    private Integer id;
    private String username;
    private String password;
    private Boolean active;
    private String email;


      @ManyToMany
      @JoinTable(name="USERS_ROLES", 
              joinColumns={@JoinColumn(name="username", referencedColumnName="username")},
               inverseJoinColumns={@JoinColumn(name="obser", referencedColumnName="obser")}

              )
    private Collection<Role>roles;
    public Collection<Role> getRoles() {
        return roles;
    }
    public void setRoles(Collection<Role> roles) {
        this.roles = roles;
    }
    public User() {
        super();
    }
    public User(Integer id,String username, String password, Boolean active, String email) {
        super();
        this.id = id;
        this.username = username;
        this.password = password;
        this.active = active;
        this.email = email;
    }
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getUsername() {
        return username;
    }
    public void setUsername(String username) {
        this.username = username;
    }
    public String getPassword() {
        return password;
    }
    public void setPassword(String password) {
        this.password = password;
    }
    public Boolean getActive() {
        return active;
    }
    public void setActive(Boolean active) {
        this.active = active;
    }
    public String getEmail() {
        return email;
    }
    public void setEmail(String email) {
        this.email = email;
    }}

**here is y Role RoleRepository:**


    public interface RoleRepository extends JpaRepository<Role, Integer> {
        List<Role> findAll();
        Role findByObser(String obser);
    }

**here is my User RoleRepository:**

    public interface UserRepository extends JpaRepository<User, Integer> {
        User findByUsername(String username);
    }


**here is my function AddRoleToUser in  my UserService:**
**When I want to add a role to a user, I use this method**

    @RequestMapping(value="/addRoleToUser")
    public User addRoleToUser(String username, String role) {
        User u = userRepository.findByUsername(username);
        Role r = roleRepository.findByObser(role);



      u.getRoles().add(r); 
              userRepository.save(u);
            return u;
    }

Unfortunately, it does not assign the role to the user and it creates an exception of type: query did not return a unique result: 3; nested exception is javax.persistence.NonUniqueResultException: query did not return a unique result: 3 then I ask if there is someone who can help me the other method like add role, add user its mace its problem

This might be because you might have more then one user with same username, so either you can set username as unique or use user_id for all the transactions or persistence.