We have a lot of contacts in our system that share the same email address. In one instance, there are 37 contacts with the same email address. For mass mailings (via CiviMail), we have turned on email de-duping checkbox here:
Administer > CiviMail > CiviMail Component Settings by ticking or unticking "CiviMail dedupes e-mail addresses by default"
Still, that one email address with 37 contacts is receiving 37 emails. It seems like it is doing so once for each contact record associated with that email. We have checked civicrm_mailing_recipients, civicrm_mailing_event_queue, and civicrm_mailing_delivered tables and confirmed that they only show one record for this email address for the mailing job in question.
We recently upgraded to Civi 4.7.25 on Joomla 3.8.1.
Any ideas on why duplicate emails are still being sent? Possibly a redux of this issue? https://issues.civicrm.org/jira/browse/CRM-19353