Our initial time-series bucket ID generation method was susceptible to collisions under conditions of coarse granularity and high bucket generation rates. This was fixed in SERVER-61194, but the fix still allows for the low-probability event that an election could cause further collisions for a period of time. That can be worked around by restarting the new primary. The hope is that we can resolve this in a more graceful way that doesn't require restarting a machine.