Mail is sent twice!

Mail is sent twice!

Been using Zoho for a while now. Installed Zoho for someone else and some weird things are happening.
Mails are being sent twice. He is using Thunderbird as an email client.

I already read about email being duplicated in the sent folder. But in my case the emails are also actually being sent twice. So people receive an exact duplicate of the email 1 or 2 seconds after the first.
So unticking the 'place a copy in Sent Folder'  doesn't resolve the issue.
No filters, no add-ons in Thunderbird. Clean install.

Please help!!