Christopher found a case where some tasks would remain unseen in the run
queue and would spontaneously appear after certain apparently unrelated
operations performed by the other thread.
It's in fact the insertion which is not correct, the node serving as the
top of duplicate tree wasn't properly updated, just like the each top of
subtree in a duplicate tree. This had the effect that after some removals,
the incorrectly tagged node would hide the underlying ones, which would
then suddenly re-appear once they were removed.
This is 1.8-specific, no backport is needed.
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