Move duplicate key parsing from bootstrap to validate/diagnose to reduce bootstrap time a bit

src/Composer/Factory.php

@@ -316,13 +316,6 @@ class Factory
                 $message = $e->getMessage() . ':' . PHP_EOL . $errors;
                 throw new JsonValidationException($message);
             }
-            $jsonParser = new JsonParser;
-            try {
-                $jsonParser->parse(file_get_contents($localConfig), JsonParser::DETECT_KEY_CONFLICTS);
-            } catch (DuplicateKeyException $e) {
-                $details = $e->getDetails();
-                $io->writeError('<warning>Key '.$details['key'].' is a duplicate in '.$localConfig.' at line '.$details['line'].'</warning>');
-            }
 
             $localConfig = $file->read();
             $localConfigSource = $file->getPath();

src/Composer/Util/ConfigValidator.php

@@ -20,6 +20,8 @@ use Composer\IO\IOInterface;
 use Composer\Json\JsonFile;
 use Composer\Pcre\Preg;
 use Composer\Spdx\SpdxLicenses;
+use Seld\JsonLint\DuplicateKeyException;
+use Seld\JsonLint\JsonParser;
 
 /**
  * Validates a composer configuration.
@@ -77,6 +79,14 @@ class ConfigValidator
             return array($errors, $publishErrors, $warnings);
         }
 
+        $jsonParser = new JsonParser();
+        try {
+            $jsonParser->parse(file_get_contents($file), JsonParser::DETECT_KEY_CONFLICTS);
+        } catch (DuplicateKeyException $e) {
+            $details = $e->getDetails();
+            $warnings[] = 'Key '.$details['key'].' is a duplicate in '.$file.' at line '.$details['line'];
+        }
+
         // validate actual data
         if (empty($manifest['license'])) {
             $warnings[] = 'No license specified, it is recommended to do so. For closed-source software you may use "proprietary" as license.';