ActivityPub: Handle clashing nicknames for the same ap id

If we get a new user (identified by ap_id) that would have the same
nickname as an existing user, give the existing user a nickname that
is prepended with the user id, as this will never clash.

This can happen when a user switches server software and that soft-
ware generates ap ids in a different way.

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -1371,6 +1371,16 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
     end
   end
 
+  def maybe_handle_clashing_nickname(nickname) do
+    with %User{} = old_user <- User.get_by_nickname(nickname) do
+      Logger.info("Found an old user for #{nickname}, ap id is #{old_user.ap_id}, renaming.")
+
+      old_user
+      |> User.remote_user_changeset(%{nickname: "#{old_user.id}.#{old_user.nickname}"})
+      |> User.update_and_set_cache()
+    end
+  end
+
   def make_user_from_ap_id(ap_id) do
     user = User.get_cached_by_ap_id(ap_id)
 
@@ -1383,6 +1393,8 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
           |> User.remote_user_changeset(data)
           |> User.update_and_set_cache()
         else
+          maybe_handle_clashing_nickname(data[:nickname])
+
           data
           |> User.remote_user_changeset()
           |> Repo.insert()