The problem is that if you load a browser window in a stream narrow,
add_message_metadata will be called for the messages in the narrowed
view before it is called for the messages going into the main view
(thus inserting them into all_msg_list), resulting in duplicate
copies of messages.
This would be mostly OK except that we call
process_message_for_recent_subjects inside add_message_metadata, and
that function assumes it is only called once on each message
(otherwise it'll double-count the message).
(imported from commit a3e7f85874100cd93a6d07684605da04d9cc80c7)