Steps to reproduce

1. Open some changeset in two tabs.
2. In one tab, approve it.
3. Try to approve it also in another tab (without refreshing the page)

Expected result

"You already approved this changeset." error message is shown and the page allows me to unapprove the changeset.

Actual result

I see a JSON message:

#!javascript

{"error": {"message": "You already approved this changeset."}}

Also, the "Approve" button is disabled and there is a spinner left spinning on it.