diff --git a/plugins/chat/app/models/chat/message.rb b/plugins/chat/app/models/chat/message.rb
index e50c794fa4f..7e642fe30b9 100644
--- a/plugins/chat/app/models/chat/message.rb
+++ b/plugins/chat/app/models/chat/message.rb
@@ -223,7 +223,7 @@ module Chat
     ]
 
     def self.cook(message, opts = {})
-      rules = MARKDOWN_IT_RULES
+      rules = MARKDOWN_IT_RULES.dup
       rules << "heading" if opts[:user_id] && opts[:user_id].negative?
 
       # A rule in our Markdown pipeline may have Guardian checks that require a