FIX: remove complicated 'chat_duplicate_message_sensitivity' site setting (#30516)

And change the "formula" to check for duplicate messages to

- no duplicate check in 1:1 DMs
- only duplicate check in group DMs / channels, for posts made by the
same user, in the past 10 seconds

Internal ref - t/144262

plugins/chat/spec/plugin_helper.rb

@@ -43,7 +43,7 @@ module ChatSystemHelpers
             chat_channel_id: channel.id,
             in_reply_to_id: in_reply_to,
             thread_id: thread_id,
-            message: Faker::Alphanumeric.alpha(number: SiteSetting.chat_minimum_message_length),
+            message: fake_chat_message,
           },
         )
 
@@ -68,6 +68,10 @@ module ChatSpecHelpers
           )
   end
 
+  def fake_chat_message
+    Faker::Alphanumeric.alpha(number: [15, SiteSetting.chat_minimum_message_length].max)
+  end
+
   def update_message!(message, text: nil, user: Discourse.system_user, upload_ids: nil)
     Chat::UpdateMessage.call(
       guardian: user.guardian,