I’m using Invoice Ninja v4.1.5 and I get the notification of my clients that they receive everything twice in their mailbox.
Does anyone has also this issue and maybe a fix? 
Can you check you aren’t running the send reminders cron more than once a day.
http://docs.invoiceninja.com/en/latest/configure.html#recurring-invoices-and-reminder-emails
Good question, but no, my cron is running once a day (morning). Also, when I receive a payment and I send them the confirmation that I’ve received their payment (checkbox) they receive them also twice :-/
Does the client have multiple contacts? The emails are sent to each checked contact email address.
Unfortunately not, only 1 e-mail adres per client and no multiple contacts connected to it.
Maybe an issue of outlook? I’m using Outlook for sending via SMTP.
You may want to try testing with a different email provider.
I’ll try 