As discovered and outlined in this thread
the function "Add record to Selected" in Webmin's BIND module fails when you try to add e.g. the following two MX records:
5 mx1.example.com 10 mx2.example.com
Webmin will complain when adding the second record, that the first already exists. Turning on "Add even if already exists", the second record gets added correctly, so it seems to be a bug only in the check for existence.