Defining duplicate factories should raise an error

[r00k]

I've been bitten a few times recently by accidentally defining two factories with the same name. Factory_girl accepts this silently. I think an error is warranted: I can't think of a scenario where users are benefited by this ability and doing it inadvertently is likely to break things.

Here's my implementation. Attempting to define a second factory with the same name raises a DuplicateDefinitionError:
http://github.com/r00k/factory_girl/tree/disallow_duplicate_factory_definitions

[stevenderocher]

This was pulled 10/30/09s.