Deduplicate ActionText::RichText embeds

Fix that an ActiveRecord::RecordNotUnique error would be raised when saving rich-text content with the same file attached multiple times.

actiontext/app/models/action_text/rich_text.rb

@@ -15,7 +15,7 @@ class RichText < ActiveRecord::Base
     has_many_attached :embeds
 
     before_save do
-      self.embeds = body.attachables.grep(ActiveStorage::Blob) if body.present?
+      self.embeds = body.attachables.grep(ActiveStorage::Blob).uniq if body.present?
     end
 
     def to_plain_text

actiontext/test/unit/model_test.rb

@@ -44,6 +44,15 @@ class ActionText::ModelTest < ActiveSupport::TestCase
     assert_equal [ActiveStorage::Attachment], message.content.embeds.map(&:class)
   end
 
+  test "embed extraction deduplicates file attachments" do
+    blob = create_file_blob(filename: "racecar.jpg", content_type: "image/jpg")
+    content = ActionText::Content.new("Hello world").append_attachables([ blob, blob ])
+
+    assert_nothing_raised do
+      Message.create!(subject: "Greetings", content: content)
+    end
+  end
+
   test "saving content" do
     message = Message.create!(subject: "Greetings", content: "<h1>Hello world</h1>")
     assert_equal "Hello world", message.content.to_plain_text