Reactivity issue when duplicating events

[st3iny]

One weird behavior: When I duplicate an event off a recurring event, I cannot configure recurrence as it says that the event is already a recurrence exception.
But when I save the duplicated event and select it again, it is suddenly an independent event and can be configured to recur.

Originally posted by @xeruf in #4300 (comment)

[max65482]

While you're at it: #4538 may be related as it also concerns the way duplicated events are represented internally.