New rule: Duplicate fields in schema

[tvvignesh]

Is your feature request related to a problem? Please describe.

When I duplicate fields in my schema and use codegen, I get errors like these:

Looks like this error is coming in from graphql-js and not codegen itself. But I don't find a rule for duplicated fields (in schema) as a rule. There is a rule for documents though.

Describe the solution you'd like
Adding a new rule for duplicated fields in schema in graphql-eslint can address this.

Describe alternatives you've considered
Alternative currently is to rely on codegen for throwing errors like these, but this can be captured in eslint itself.

[dimaMachina]

Hi @tvvignesh, currently buildSchema from graphql throws an error when find a duplicated field for ObjectTypeDefinition, InterfaceTypeDefinition, EnumTypeDefinition, InputObjectTypeDefinition kinds and same for their extensions ObjectTypeExtension... etc.
I agree with you, we can validate duplication fields before building our schema, the new rule or new option for the existing rule avoid-duplicate-fields will be added 😉

UPDATE:
We have already rules from graphql-js that checks for duplication fields, like unique-field-definition-names, unique-input-field-names, unique-enum-value-names and they work not as expected, when you have split schema for multiple files, I'll try to fix this