Duplicate Calendar Invites Sent

Beginning 12/13/18 we got a report from one of our end users that Calendar invite emails were being sent multiple times to the recipients (both internal and external). 

The recipient would report receiving the invite many times (10+ in many cases) but the invite email only shows up once in the meeting creators Outlook Sent Items folder.  We verified through message tracker that the invite is actually being sent by O365 several times so it's not something wrong with the recipients side recreating them.

We now have several users who are reporting this issue as of 12/14.

We thought it might be Outlook or Outlook plugin related but we've kind of discounted that because even with Outlook closed the invite emails continue to be sent.

Is anyone else seeing this type of activity?  We have a ticket opened with O365 support but it hasn't yielded any positive results yet.  Thoughts?