FIX: Never return the same reply more than once via reply_ids

If our reply tree somehow ends up with cycles or other odd
structures, we only want to consider a reply once, at the first
level in the tree that it appears.

spec/models/post_spec.rb

@@ -956,6 +956,11 @@ describe Post do
       expect(p1.reply_ids).to be_blank
     end
 
+    it "doesn't include the same reply twice" do
+      PostReply.create!(post: p4, reply: p1)
+      expect(p1.reply_ids.size).to eq(4)
+    end
+
     it "does not skip any replies" do
       expect(p1.reply_ids(only_replies_to_single_post: false)).to eq([{ id: p2.id, level: 1 }, { id: p4.id, level: 2 }, { id: p5.id, level: 3 }, { id: p6.id, level: 2 }])
       expect(p2.reply_ids(only_replies_to_single_post: false)).to eq([{ id: p4.id, level: 1 }, { id: p5.id, level: 2 }, { id: p6.id, level: 1 }])