Change the key of a global key (indicating the @emotion/react has bee…

…n already loaded) to include the major version (#1981)

.changeset/long-geckos-raise.md

@@ -0,0 +1,5 @@
+---
+'@emotion/react': patch
+---
+
+Changed the key of the global flag that helps us identify that `@emotion/react` has been loaded more than once to include the current major version of `@emotion/react`.

packages/react/src/index.js

@@ -1,4 +1,5 @@
 // @flow
+import pkg from '../package.json'
 export { withEmotionCache, CacheProvider } from './context'
 export { jsx } from './jsx'
 export { Global } from './global'
@@ -14,7 +15,7 @@ if (process.env.NODE_ENV !== 'production') {
 
   if (isBrowser && !isJest) {
     const globalContext = isBrowser ? window : global
-    const globalKey = '__EMOTION_REACT__'
+    const globalKey = `__EMOTION_REACT_${pkg.version.split('.')[0]}__`
     if (globalContext[globalKey]) {
       console.warn(
         'You are loading @emotion/react when it is already loaded. Running ' +