We are running WordPress 4.4.2, CiviCRM 4.6.12, and the CiviCRM WordPress Member Sync and CiviCRM WordPress Profile Sync plugins.
Whenever someone logs in with a WordPress user ID / email that is not their primary CiviCRM contact email, a new CiviCRM contact is created. The WP email may be listed as their "home" / "work" / "billing" / etc. email in CiviCRM but a new contact is still created.
In the newly created CiviCRM contact, the email address is set as "home" and "primary". The next time they login, a new contact is created, again with the email set as "home". We end up with a lot of duplicate contacts!
Is this a plugin issue, or a CiviCRM issue?
It seems similar to the question here: Wordpress duplicates my contacts