Creates recurring events twice randomly

[HuidaeCho]

When a new CalDAV calendar is added, some recurring events are synced (?) and created twice randomly, but not all of them. For example, in the first sync, event 1 was created twice and event 2 was OK. After deleting the two events from the roundcube calendar (not from the CalDAV server), another sync can create event 2 twice and maybe event 1 as well. I'm not sure what triggers sync of these duplicate events twice. What's more interesting is that it's not all events in one calendar. Only some of them.