Emails sent twice

First of all, thank you for maintaining this highly useful plugin.
I have an issue that sometimes occurs on different client websites: randomly, emails are sent twice. This has happened with emails sent via WooCommerce (order confirmations) or Contact Form 7 plugin.
The emails are sent within seconds from each other, and I can pretty much rule out the possibility that the CF7 was submitted twice because I generate a unique ID with each contact request, and the ID in the duplicate emails was the same.
Is it possible that the plugin sends emails twice in some cases, or does the issue maybe lie with Mandrill itself?
Thanks for your support.