Users receive the same notification lots of times over a longer period, at a rate of about one per day. The notification is about an old edit (often several years old, although always the last edit to the page), with no obvious reason for being triggered now. Seems specific to plwiki.
Some people report only getting these spurious notifications via email (despite having configured both types) and seeing them as already marked read in Special:Notifications, others seem to get the web notifications as well. Multiple notification types (talk page message, mention), multiple edit types (revert, page move). Marking some of the notifications as read doesn't stop new ones from being triggered.