SortedVector considered an appended element equal to the last one to
break the ordering, marking the vector as unsorted. A non-decreasing
sequence is sorted, so only a strictly smaller element has to trigger
the marker. Equal keys are common: child sample vectors receive
identical timestamps whenever a recursive call stack contains the same
call site twice, which flipped the vectors to unsorted on virtually
every recursive workload and caused the lazy sort in GetChildSamples to
run over and over again while holding the data lock.
The lazy sort machinery handles duplicate keys correctly, as both the
prefix and tail merge windows are computed with lower bounds.
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