FIX: keep unique post checks separate for PMs vs topics (#17272)

This allows for people to use PMs for drafting and then post them on topics.

Co-authored-by: Alan Guo Xiang Tan <gxtan1990@gmail.com>

app/models/post.rb

@@ -248,7 +248,7 @@ class Post < ActiveRecord::Base
 
   # The key we use in redis to ensure unique posts
   def unique_post_key
-    "unique-post-#{user_id}:#{raw_hash}"
+    "unique#{topic&.private_message? ? "-pm" : ""}-post-#{user_id}:#{raw_hash}"
   end
 
   def store_unique_post_key