During the week of September 9, 2018 I started to notice duplicate messages that were sent to an IIS SMTP server to relay onto Office 365 for delivery.  I have setup IIS SMTP logging to see if the software or device (document scanner) or security hardware or software that send mail were generating multiple messages.  In the IIS SMTP log it shows only one message is received by IIS SMTP and only one message is sent on to Office 365 SMTP Server smtp.office365.com.

Looking at Reddit I saw a discussion of multiple people running into this same issue. One person contacting Office 365 Support was able get them to admit they were aware of the issue and it would take 4-6 weeks for it to be resolved from 9/24/2018.

https://www.reddit.com/r/sysadmin/comments/9hqrbj/duplicate_emails_coming_from_iis_relay_to_o365/

On a somewhat related note during the week of 9/16/2018 issues with delivery from Sharp Multifunction Copiers started to be blocked due to the reply-to setting in the device.  The error message that would occur was smtp;554 5.2.0 STOREDRV.Submission.Exception:SendAsDeniedException.MapiExceptionSendAsDenied. The article below also covers how to properly support TLS 1.2 on Sharp MFC if your directly sending to Office 365 SMTP.

https://community.spiceworks.com/topic/2126027-sharp-mx-2640n-scan-to-email-with-office365