Prevent double insertions.

test/web/activity_pub/activity_pub_test.exs

@@ -7,6 +7,13 @@ defmodule Pleroma.Web.ActivityPub.ActivityPubTest do
   import Pleroma.Factory
 
   describe "insertion" do
+    test "returns the activity if one with the same id is already in" do
+      activity = insert(:note_activity)
+      {:ok, new_activity}= ActivityPub.insert(activity.data)
+
+      assert activity == new_activity
+    end
+
     test "inserts a given map into the activity database, giving it an id if it has none." do
       data = %{
         "ok" => true