CiviMail sending multiple copies to test address and when submitted

Testing a CiviMail message, multiple emails are sent to the test message address. Emails are sent in batches and continue until the draft message is deleted. Dozens of messages can be sent. Seems to stop after a period of time and 50+ messages are sent.

When message is submitted for broadcast, one of the addresses in the list receives multiple emails - 10 to 15. Other emails either are not sent or only receive one message. Seems to be inconsistent.

Other facts:
Same behavior when using SMTP or mail()
Emails sent by Drupal are OK
Test email settings message is OK
mail log shows individual messages with different headers.
This error appears in mail log - periodically:
did not issue MAIL/EXPN/VRFY/ETRN during connection to MTA

I am going to roll back to 4.7.7 to see if the problem is in that version as well.

Update: Problem is not evident after rollback to 4.7.7. Have not (yet) encountered the issue in other instances which are on 4.7.8.