Add warning if multiple versions are running (addresses #1470)

packages/core/src/index.js

@@ -6,3 +6,18 @@ export { keyframes } from './keyframes'
 export { ClassNames } from './class-names'
 export { ThemeContext, useTheme, ThemeProvider, withTheme } from './theming'
 export { default as css } from './css'
+
+if (process.env.NODE_ENV !== 'production') {
+  const isBrowser = typeof document !== 'undefined'
+  const globalKey = '__EMOTION_CORE__'
+  const globalContext = isBrowser ? window : global
+  if (globalContext[globalKey]) {
+    console.warn(
+      'You are loading @emotion/core when it is already loaded. Running' +
+        'multiple instances may cause problems. This can happen if multiple ' +
+        'versions are used, or if multiple builds of the same version are ' +
+        'used.'
+    )
+  }
+  globalContext[globalKey] = true
+}

packages/utils/src/index.js

@@ -1,7 +1,7 @@
 // @flow
 import type { RegisteredCache, EmotionCache, SerializedStyles } from './types'
 
-let isBrowser = typeof document !== 'undefined'
+const isBrowser = typeof document !== 'undefined'
 
 export function getRegisteredStyles(
   registered: RegisteredCache,