How to avoid adding duplicates in a many-to-many relationship table in SQLAlchemy - python?

I am dealing with a many-to-many relationship with sqlalchemy. My question is how to avoid adding duplicate pair values in a many-to-many relational table.

To make things clearer, I will use the example from the official SQLAlchemy documentation.

Base = declarative_base()

Parents2children = Table('parents2children', Base.metadata,                                                                                                                                                                                                     
  Column('parents_id', Integer, ForeignKey('parents.id')),                                                                                                                                                                                                       
  Column('children_id', Integer, ForeignKey('children.id'))
)

class Parent(Base):
    __tablename__ = 'parents'
    id = Column(Integer, primary_key=True)
    parent_name = Column(String(45))
    child_rel = relationship("Child", secondary=Parents2children, backref= "parents_backref") 

    def __init__(self, parent_name=""):
        self.parent_name=parent_name
    def __repr__(self):
        return "<parents(id:'%i', parent_name:'%s')>" % (self.id, self.parent_name)

class Child(Base):
    __tablename__ = 'children'
    id = Column(Integer, primary_key=True)
    child_name = Column(String(45))

    def __init__(self, child_name=""):
        self.child_name= child_name
    def __repr__(self):
        return "<experiments(id:'%i', child_name:'%s')>" % (self.id, self.child_name)

###########################################

def setUp():
    global Session
    engine=create_engine('mysql://root:root@localhost/db_name?charset=utf8', pool_recycle=3600,echo=False)
    Session=sessionmaker(bind=engine)

def add_data():
    session=Session()
    name_father1=Parent(parent_name="Richard")
    name_mother1=Parent(parent_name="Kate")
    name_daughter1=Child(chil