Duplicate sent 6 weeks apart

Hi. We have an experience where an email was sent twice (same content) from our WordPress site which uses Easy SMTP. The first email sent was deliberately made from a WordPress form by a customer.

6 weeks later, the same email was received by our inbox. Upon calling the customer, he said that they did not fill out anything from our website. Our EasySMTP mailer settings is “Other SMTP” and linked to our OFfice 365 SMTP account. We are using a free version

I also checked with Office 365 support and they said that on their end, the email was sent twice and not stuck on their end. They suggested to check with EasySMTP plugin.

My question is:
1. Is there a known issue that an email gets stuck and sent out on a random time?

2. For free issues, is there an internal log we could look at?

Thank you in advance!