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In networkstatus.c we have:

/** A time before which we shouldn't try to replace the current consensus:
 * this will be at some point after the next consensus becomes valid, but
 * before the current consensus becomes invalid. */
static time_t time_to_download_next_consensus = 0;

which we use in update_consensus_networkstatus_downloads() to decide whether to break out:

  if (time_to_download_next_consensus > now)
    return; /* Wait until the current consensus is older. */

The result is that we don't even try to fetch a microdesc consensus if the ns consensus we have is recent enough.