Remove doubled 'to' recipients.

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -24,7 +24,7 @@ def create(to, actor, context, object, additional \\ %{}, published \\ nil, loca
 
     activity = %{
       "type" => "Create",
-      "to" => to,
+      "to" => to |> Enum.uniq,
       "actor" => actor.ap_id,
       "object" => object,
       "published" => published,

test/web/activity_pub/activity_pub_test.exs

@@ -40,6 +40,13 @@ test "adds an id to a given object if it lacks one and inserts it to the object
     end
   end
 
+  describe "create activities" do
+    test "removes doubled 'to' recipients" do
+      {:ok, activity} = ActivityPub.create(["user1", "user1", "user2"], %User{ap_id: "1"}, "", %{})
+      assert activity.data["to"] == ["user1", "user2"]
+    end
+  end
+
   describe "fetch activities for recipients" do
     test "retrieve the activities for certain recipients" do
       {:ok, activity_one} = ActivityBuilder.insert(%{"to" => ["someone"]})