Error: Field "My.x" already exists in the schema?

Question

Node.js

it can schema

index.js

import { gql } from "apollo-server-express";
import My from "./My";

const linkSchema = gql`
  type Query {
    my: My
  }
  type My {
    _: Boolean
  }
`;
export default [linkSchema, My];

My.js

import { gql } from "apollo-server-express";

export default gql`
  extend type My {
    x: Int
  }
`;

package.json

"dependencies": {
    "apollo-server": "^2.14.4",
    "apollo-server-express": "^2.14.4",
    "express": "^4.17.1",
    "graphql": "^15.1.0",
  }

when GraphQL version 14 everything works. but when changing to 15 I get an error.

Error: Field "My.x" already exists in the schema. It cannot also be defined in this type extension

what is the reason. How can I fix this error? Thanks in advance. Thanks in advance for your answers.