Use race-condition free mass follow.

lib/pleroma/user.ex

@@ -309,20 +309,21 @@ defmodule Pleroma.User do
   @doc "A mass follow for local users. Ignores blocks and has no side effects"
   @spec follow_all(User.t(), list(User.t())) :: {atom(), User.t()}
   def follow_all(follower, followeds) do
-    following =
-      (follower.following ++ Enum.map(followeds, fn %{follower_address: fa} -> fa end))
-      |> Enum.uniq()
+    followed_addresses = Enum.map(followeds, fn %{follower_address: fa} -> fa end)
 
-    {:ok, follower} =
-      follower
-      |> follow_changeset(%{following: following})
-      |> update_and_set_cache
+    q =
+      from(u in User,
+        where: u.id == ^follower.id,
+        update: [set: [following: fragment("array_cat(?, ?)", u.following, ^followed_addresses)]]
+      )
+
+    {1, [follower]} = Repo.update_all(q, [], returning: true)
 
     Enum.each(followeds, fn followed ->
       update_follower_count(followed)
     end)
 
-    {:ok, follower}
+    set_cache(follower)
   end
 
   def follow(%User{} = follower, %User{info: info} = followed) do