ical-recurrence: test that the recurrence set has no two instances with the same start
recurEvents unions sets of EventOccurrence, which deduplicates on
(start, end) pairs rather than on the start alone. A period-valued
RDATE whose start coincides with an instance the RRULE generates
therefore yields two instances at the same instant with different ends.
RFC 5545 says the period-valued form of RDATE is how the duration of a
specific recurrence is modified, so the RDATE should win here.
The same can happen without any RDATE at all: recurEvents inserts the
DTSTART occurrence with the original end while the rule separately
regenerates it through resolveEndOrDurationDate, and those two agree
only when unresolveUTCTimeR . resolveLocalTimeR round-trips, which it
does not for a DTEND whose local time falls in a daylight saving time
gap.