Forbid duplicate type definitions

[turadg]

A GraphQL schema can't have more than one definition of the same type.

Facebook runs an integration test to prevent this:
#223 (comment)

With this PR the buildASTSchema step throws, causing any tests that load the schema to fail.

[turadg]

@leebyron this is the bug we discussed at React Conf.

[leebyron]

Constructing a GraphQLSchema should throw this error, correct?

Also, in the context of an AST document, it would be great to throw GraphQLError with reference to the problem AST nodes.

[wincent]

Thanks @turadg. This looks reasonable to me and in line with the spec which says:

All types within a GraphQL schema must have unique names. No two provided types may have the same name.

And schema construction is obviously the right time to throw the error too. So I'll merge this. Would you like to follow up and make @leebyron's suggested change of throwing a GraphQLError instead? Some tools may appreciate the metadata.

[turadg]

Thanks @wincent . I haven't encountered that case but if I do I bet I'll be back with another PR. :)