title: sqlalchemy Error creating backref on relationship
body: <p>I have two very simple models. In my <code>Post</code> model there are supposed to be two relationships into the <code>User</code> table. One is for the owner of the post and one is for the last editor of the post. They can be different values, but both refer to the same <code>User</code> table.</p>
  
  <p>My models are set up like this</p>
  
  <pre><code>class Post(Base):
      last_editor_id = Column(BigInteger, ForeignKey('users.id'), nullable=True)
      last_editor = relationship('User', backref='posts', foreign_keys=[last_editor_id])
      owner_id = Column(BigInteger, ForeignKey('users.id'), nullable=False, index=True)
      owner = relationship('User', backref='posts', foreign_keys=[owner_id])
  
  class User(Base):
      '''This represents a user on the site'''
      __tablename__ = 'users'
      id = Column(BigInteger, primary_key=True, unique=True)
      name = Column(BigInteger, nullable=False)
  </code></pre>
  
  <p>When I attempt to create these models though, I get the following error</p>
  
  <pre><code>sqlalchemy.exc.ArgumentError: Error creating backref 'posts' on relationship 'Post.owner': property of that name exists on mapper 'Mapper|User|users'
  </code></pre>
  
  <p>How do I correct this so that I can maintain both forgeign keys in the <code>Post</code> model?</p>