You’ll want to see the above post^ This patch to swiftmailer should solve your duplicate send issue. works well for us, and we were able to run multiple sending crons at the same time 
What we’re using for now:
sudo php /srv/mautic/bin/console mautic:emails:send --lock_mode=flock --lock-name="1" & sleep 2 & sudo php /srv/mautic/bin/console mautic:emails:send --lock_mode=flock --lock-name="2" & sleep 2 & sudo php /srv/mautic/bin/console mautic:emails:send --lock_mode=flock --lock-name="3"
Using the above code, we benchmarked some email tests. This code runs 3 send commands at the same time, which is about 18 emails per second to our SMTP sever… which is too fast. We’re going to only run 2, or about 12.5 emails per second.