Warn (or solve) if there are duplicate child class field names

[mnieber]

The following does not work because the name "request_message" is duplicated in two child classes of the base class Message, which produces a conflict later on when querying the database.

class Message(TypedModel):
pass

class AcceptMessage(Message):
request_message = models.ForeignKey('self', null=True, blank=True, related_name='+')

class DeclineMessage(Message):
request_message = models.ForeignKey('self', null=True, blank=True, related_name='+')

This can be solved by moving request_message to the base class but this is not always what you want (perhaps a third class inheriting from Message does not have a request_message field).

It would be nice if TypedModel would at least raise an exception in this case so that the programmer becomes aware of the problem.