If an update on the zone is received by NSD, and an AXFR of the zone is still running at that time, the AXFR transfer of the zone starts from the beginning of the zone again.
Therefore, certainly for a bigger zone, within the AXFR output, the first entries of the zone are shown mutiple times.
The AXFR output displays the following message when this happens:
;; communications error to <ipaddress>#53: end of file