Describe the bug
GraphQL Java's validation allows an extension type to define a field that has already been defined in the base class, if the field has the same type and arguments in both places.

To Reproduce
In this example, the field name is defined in the base type Human and then defined again in the extension type. This should generate a validation error, but it doesn't.

            schema {
              query: Human
            }
            
            type Human {
                id: ID!
                name: String!
            }
            extend type Human {
                name: String!
                friends: [String]
            }