Contact import update warnings/error on matching contact - not added to group

Behavior:

The following warnings occur when importing existing contacts (update option, deduping on email)

• Warning: explode() expects parameter 2 to be string, array given in //wp-content/plugins/civicrm/civicrm/CRM/Contact/Import/Parser/Contact.php on line 704
• Warning: array_pop() expects parameter 1 to be array, null given in //wp-content/plugins/civicrm/civicrm/CRM/Contact/Import/Parser/Contact.php on line 706
• Warning: Cannot modify header information - headers already sent by (output started at //wp-content/plugins/civicrm/civicrm/CRM/Contact/Import/Parser/Contact.php:704) in //wp-content/plugins/civicrm/civicrm/CRM/Utils/System/Base.php on line 912

The existing contacts are getting updated but they are not getting added to the specified group.  (New contacts in the same import are created and added to the group correctly).

A strange log is created in the matching contact's record with references to other contacts in the import.

I am running 4.7.19 in WP and I upgraded recently from 4.6.20 where the client did not have this problem.

I have reproduced the behavior on the civicrm demo/testing sandbox :/

Related JIRA Issue?:

The comment above the culprit section of code (lines 704-706 in contact.php)

   "see CRM-10433 might return comma separate list of all dupes"

refers to https://issues.civicrm.org/jira/browse/CRM-10433 titled "import fails when updating on duplicate, adding to group, and multiple duplicates exist"