FIX: prevents duplicate attachments in incoming emails (#25986)

## What?

Depending on the email software used, when you reply to an email that has some attachments, they will be sent along, since they're part of the embedded (replied to) email.

When Discourse processes the reply as an incoming email, it will automatically add all the (valid) attachments at the end of the post. Including those that were sent as part of the "embedded reply".

This generates posts in Discourse with duplicate attachments 🙁

## How?

When processing attachments of an incoming email, before we add it to the bottom of the post, we check it against all the previous uploads in the same topic. If there already is an `Upload` record, it means that it's a duplicate and it is _therefore_ skipped.

All the inline attachments are left untouched since they're more likely new attachments added by the sender.

lib/email/receiver.rb

@@ -1359,6 +1359,12 @@ module Email
     def add_attachments(raw, user, options = {})
       raw = raw.dup
 
+      previous_upload_ids =
+        UploadReference
+          .where(target_id: Post.where(topic_id: options[:topic_id]).select(:id))
+          .pluck(:upload_id)
+          .uniq
+
       rejected_attachments = []
       attachments.each do |attachment|
         tmp = Tempfile.new(["discourse-email-attachment", File.extname(attachment.filename)])
@@ -1384,10 +1390,10 @@ module Email
                 end
               elsif raw[/\[image:[^\]]*\]/i]
                 raw.sub!(/\[image:[^\]]*\]/i, UploadMarkdown.new(upload).to_markdown)
-              else
+              elsif !previous_upload_ids.include?(upload.id)
                 raw << "\n\n#{UploadMarkdown.new(upload).to_markdown}\n\n"
               end
-            else
+            elsif !previous_upload_ids.include?(upload.id)
               raw << "\n\n#{UploadMarkdown.new(upload).to_markdown}\n\n"
             end
           else

spec/lib/email/receiver_spec.rb

@@ -756,6 +756,16 @@ RSpec.describe Email::Receiver do
       MD
     end
 
+    it "tries not to repeat duplicate attachments" do
+      SiteSetting.authorized_extensions = "jpg"
+
+      expect { process(:logo_1) }.to change { UploadReference.count }.by(1)
+      expect(topic.posts.last.raw).to match %r{upload://}
+
+      expect { process(:logo_2) }.not_to change { UploadReference.count }
+      expect(topic.posts.last.raw).not_to match %r{upload://}
+    end
+
     it "works with removed attachments" do
       SiteSetting.authorized_extensions = "jpg"