Order notification sent twice

After a customers place an order and the transaction is completed succesfully they receive the order notification twice. Emails are received wihtin a few minutes after eachother. Webshop is created in 4.0.3.

Any advice is welcome.

nevermind, sorry misunderstood your question.

I am on 3.0.x. Mine does this all the time.

Posted the problem to the helpdesk. They said it was due to the payment service provider software. Developer of PSP software could not find an issue.