diff --git a/src/Symfony/Component/Translation/MessageCatalogue.php b/src/Symfony/Component/Translation/MessageCatalogue.php
index 429a3c687747..123a9f567019 100644
--- a/src/Symfony/Component/Translation/MessageCatalogue.php
+++ b/src/Symfony/Component/Translation/MessageCatalogue.php
@@ -154,9 +154,16 @@ public function replace(array $messages, string $domain = 'messages')
     public function add(array $messages, string $domain = 'messages')
     {
         if (!isset($this->messages[$domain])) {
-            $this->messages[$domain] = $messages;
-        } else {
-            foreach ($messages as $id => $message) {
+            $this->messages[$domain] = [];
+        }
+        $intlDomain = $domain;
+        if (false === mb_strpos($intlDomain, self::INTL_DOMAIN_SUFFIX)) {
+            $intlDomain .= self::INTL_DOMAIN_SUFFIX;
+        }
+        foreach ($messages as $id => $message) {
+            if (isset($this->messages[$intlDomain]) && \array_key_exists($id, $this->messages[$intlDomain])) {
+                $this->messages[$intlDomain][$id] = $message;
+            } else {
                 $this->messages[$domain][$id] = $message;
             }
         }