add warning when Yjs was already imported

src/index.js

@@ -1,3 +1,4 @@
+/** eslint-env browser */
 
 export {
   Doc,
@@ -84,3 +85,25 @@ export {
   diffUpdate,
   diffUpdateV2
 } from './internals.js'
+
+const glo = /** @type {any} */ (typeof window !== 'undefined'
+  ? window
+  : typeof global !== 'undefined' ? global : {})
+const importIdentifier = '__ $YJS$ __'
+
+if (glo[importIdentifier] === true) {
+  /**
+   * Dear reader of this error message.
+   *
+   * If you see this message, please make sure that you only import one version of Yjs. In many cases,
+   * your package manager installs two versions of Yjs that are used by different packages of your project.
+   * Another reason for this message is that some parts of your project use the commonjs version of Yjs
+   * and others use the EcmaScript version of Yjs.
+   *
+   * This very often leads to issues that are hard to debug. We often need to do constructor checks,
+   * e.g. `struct instanceof GC`. If you importent different versions of Yjs, it is impossible for us to
+   * do the constructor checks anymore - which might break the CRDT algorithm.
+   */
+  console.warn('Yjs was already imported. Importing different versions of Yjs often leads to issues.')
+}
+glo[importIdentifier] = true