add check for iterations and browser

lib/cli/cli.js

@@ -19,6 +19,15 @@ function validateInput(argv) {
   if (argv.headless && (argv.video || argv.visualMetrics)) {
     return 'You cannot combine headless with video/visualMetrics because they need a screen to work.';
   }
+
+  if (Array.isArray(argv.browsertime.iterations)) {
+    return 'Ooops you passed number of iterations twice, remove one of them and try again.';
+  }
+
+  if (Array.isArray(argv.browser)) {
+    return 'You can only run with one browser at a time.';
+  }
+
   return true;
 }