A groups, contact or user isn't syncing to O365. There are a number of ways that you could be alerted to about this.
You will could get an error message on the 365 connector like this:
The 365 connector may not be updating:
Or you may say an error email from Azure:
The customer has either created an object and given it an email address or a proxy address of a value which already exists on an object in 365.
If de - provisioning is OFF for the customer then they may have deleted an account from AD but it stays in 365, when they come to recreate it they may give it the same email address resulting in the above error.
First you need to find the duplicate. You can do this by logging into Exchange PowerShell using the attached script (you can use standard Windows PowerShell).
The script will prompt you for 365 admin credentials, obtain these from PMP.
What you need to do then is use the command get-recipient 'insert email address' and this will tell you what object in 365 is due to receive email on that address.
An example here is a contact at Dundalk which is saying 'add' on the B connector in FIM. We need to find out what object in 365 has either the email address or one of the proxy addresses assigned to it in 365.
From the screenshot above you can see the contact has 4 proxy addresses:
You can see that there is already a contact in 365 with those email addresses assigned to it, so the one in 365 needs to be removed or the customer needs to remove or rename the email addresses on prem.
To pull up more details for the account use the |fl switch.
To delete contacts or groups from 365 see this article: