Seen flags is already used in multi-device case. If you download the message in one app (e.g. Android), read it, then open another app (e.g. Desktop), the message should be marked as seen right after downloading. dc_receive_imf procedure takes seen flag into account. But if you download the message in both apps first, then read the message in one app, the other app will not notice that Seen flag has been set on the message because the message is already downloaded. It should, however, get unsolicited FETCH response if it is online, but Delta Chat currently ignores it. Processing unsolicited FETCH responses and refetching message flags in this case is a good idea and should be done, but will not work if the second device is online. AFAIK it’s only possible to get such updates after an offline period with QRESYNC IMAP extension or by refetching message flags each time after reconnecting: