Don't save double tags in AP objects we create

This crashed Mastodon workers.

test/web/common_api/common_api_test.exs

@@ -0,0 +1,13 @@
+defmodule Pleroma.Web.CommonAPI.UtilsTest do
+  use Pleroma.DataCase
+  alias Pleroma.Web.CommonAPI
+
+  import Pleroma.Factory
+
+  test "it de-duplicates tags" do
+    user = insert(:user)
+    {:ok, activity} = CommonAPI.post(user, %{"status" => "#2hu #2HU"})
+
+    assert activity.data["object"]["tag"] == ["2hu"]
+  end
+end