Originally reported here: http://wp-event-organiser.com/forums/topic/identical-entries-appearing-twice-in-list-deleted-but-both-still-there/#post-23262

Deleting all the occurrences of an event (i.e. via the admin calendar) causes the event to disappear from the event admin list and calendar, and causes its page to 404. However, it still appears in the event list shortcode (with no date).

This appears to have been introduced in 3.1.0. The fix should allow such events to be visible in the admin screen, and deleting an event's last remaining occurrence should probably delete the entire event.