For a query shape that has never been run before, it is not optimal to cache a plan if it ties with another, because not enough information is available to conclusively rule out either plan. However, for a query shape that generates two plans that are guaranteed to always tie, it is useless to run a trial for those two plans on every query. A better solution is to cache an arbitrary plan after it ties N times in a row with the winning plan.

Related: SERVER-15225.