packages/babel-polyfill/src/index.js

import "./noConflict";

import global from "core-js/library/fn/global";

if (global._babelPolyfill && typeof console !== "undefined" && console.warn) {
  console.warn(
    "@babel/polyfill is loaded more than once on this page. This is probably not desirable/intended " +
      "and may have consequences if different versions of the polyfills are applied sequentially. " +
      "If you do need to load the polyfill more than once, use @babel/polyfill/noConflict " +
      "instead to bypass the warning.",
  );
}

global._babelPolyfill = true;