Now when you create an app and enable the append dataset configuration (beta), you’re less likely to see duplicate records when the dataflow runs. This improvement reduces the chances of reaching your row count limit.
Where: This change applies to Lightning Experience in Enterprise, Performance, Unlimited, and Developer editions.
Who: This change is available to customers who purchased Salesforce Shield or Salesforce Event Monitoring add-on subscriptions.
Streamline Dataset Updates for the Event Monitoring Analytics App (Beta) (salesforce.com)