Gracefully fail on window.dataLayer name conflict

Author: Peripheral1994

lib/index.js

@@ -30,6 +30,12 @@ var GTM = module.exports = integration('Google Tag Manager')
  */
 
 GTM.prototype.initialize = function() {
+  // If window.dataLayer already exists and isn't an array, fail gracefully.
+  if (window.dataLayer && !Array.isArray(window.dataLater)) {
+    console.error('window.dataLayer already exists - not running Segment<>GTM destination')
+    return;
+  }
+  
   push({ 'gtm.start': Number(new Date()), event: 'gtm.js' });
 
   if (this.options.environment.length) {