In terms of blocking duplicates, we only have a validation for the following unique ID's:
Source Client Order ID
Client Custom Order ID
For Shopify Clients: Source Client Order ID's are automatically sent from Shopify with valid orders. This ID is a 13-digit numerical string that can also be found in the Shopify order page's URL.
For API clients: These ID's must be generated your side and sent within the order's payload.
Only one instance of these ID's on one of your orders can exist at a time so any duplicates containing identical information would be rejected.