Detect require overrides, fixes #2503

src/Composer/Util/ConfigValidator.php

@@ -108,6 +108,11 @@ class ConfigValidator
             $warnings[] = "The package type 'composer-installer' is deprecated. Please distribute your custom installers as plugins from now on. See http://getcomposer.org/doc/articles/plugins.md for plugin documentation.";
         }
 
+        $requireOverrides = array_intersect_key($manifest['require'], $manifest['require-dev']);
+        if (!empty($requireOverrides)) {
+            $warnings[] = implode(', ', array_keys($requireOverrides)). " is required both in require and require-dev, this can lead to unexpected behavior";
+        }
+
         try {
             $loader = new ValidatingArrayLoader(new ArrayLoader());
             if (!isset($manifest['version'])) {