diff --git a/swh/web/common/highlightjs.py b/swh/web/common/highlightjs.py
--- a/swh/web/common/highlightjs.py
+++ b/swh/web/common/highlightjs.py
@@ -441,7 +441,8 @@
 
             if lang:
                 for lang_mime_type in lang_mime_types:
-                    _mime_type_to_hljs_language[lang_mime_type] = lang
+                    if lang_mime_type not in _mime_type_to_hljs_language:
+                        _mime_type_to_hljs_language[lang_mime_type] = lang
 
 
 def get_hljs_language_from_filename(filename):