I'm sending a mass mailing using the Email Marketing app in Odoo 11 (community edition). There are 4000 recipients on the list but it sends 7000 emails, duplicating some of the mails sent.

I select the addresses from the contact list using a filter. There aren't any duplicate contacts. The Mass Mailing report shows 7k mails sent from 4k, so I think Odoo is sending it twice to some contacts because it's clear that there're 4k recipients, but it sends 7k emails.

I don't use any third part module or customizations.

How can this be solved? It's strange because it doesn't duplicate all of the mails and never sends more than two to the same recipient. I can't find any logical patern in the app's behavior to figure out what the problem is.

Thank you.