You will need to provide more information about your setup. 

What is the source? Are you creating users in ServiceNow and integrating to AD, or is AD the source and are you updating ServiceNow? If you try to create users that already exist in AD (active), then you will run into the issue that AD doesn't accept it. But we need to know more about what is happening.

It sounds like a simple: we are trying to create a user that already has an account in AD. We don't know where these accounts are coming from.