Prevent double insertions.

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -8,6 +8,9 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
     |> Map.put_new_lazy("id", &generate_activity_id/0)
     |> Map.put_new_lazy("published", &make_date/0)
 
+    with %Activity{} = activity <- Activity.get_by_ap_id(map["id"]) do
+      {:ok, activity}
+    else _e ->
       map = if is_map(map["object"]) do
         object = Map.put_new_lazy(map["object"], "id", &generate_object_id/0)
         Repo.insert!(%Object{data: object})
@@ -18,6 +21,7 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
 
       Repo.insert(%Activity{data: map, local: local})
     end
+  end
 
   def create(to, actor, context, object, additional \\ %{}, published \\ nil, local \\ true) do
     published = published || make_date()

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