Warn on duplicate polymorphic_identity

Currently, reuse of a value for polymorphic_identity just overwrites the old mapping. This can lead hard to debug failiures (queried instances having unexpected types):

from sqlalchemy import create_engine, Column, Integer, String, ForeignKey
from sqlalchemy.orm import Session
from sqlalchemy.ext.declarative import declarative_base


class Spam(declarative_base()):

    __tablename__ = 'spam'

    id = Column(Integer, primary_key=True)

    kind = Column(String)

    __mapper_args__ = {'polymorphic_on': kind, 'polymorphic_identity': 'spam'}


class Eggs(Spam):

   __tablename__ = 'eggs'

   id = Column(Integer, ForeignKey('spam.id'), primary_key=True)

   __mapper_args__ = {'polymorphic_identity': 'spam'}  # warn here


engine = create_engine('sqlite://')

Spam.metadata.create_all(engine)

session = Session(engine)

session.add(Spam())
session.flush()
spam = session.query(Spam).one()

assert type(spam) is Spam  # fails

Provided there is no use case for overwriting, it would be nice to make the user aware of such misconfiguration at an early development stage.

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