A business has a proprietary Order Management System (OMS) that creates orders from their website and fulfills the orders. When the order is created in the OMS, an integration also creates an order re

Ensuring that the order number generated by the OMS is unique is a fundamental data quality requirement. If the source system itself guarantees uniqueness, it simplifies the process for Salesforce to reliably identify and prevent duplicate records.

While using the order number from the OMS as an External ID is crucial for matching, merely designating a field as an External ID does not, by itself, prevent duplicate inserts unless the field is also marked as `Unique` in the schema definition, or the integration uses an `upsert` operation.

Using the email on the contact record as an external ID would help identify or relate contacts, but it has no bearing on uniquely identifying or preventing duplicate *order* records.

A `before insert` trigger on the Order object can query for existing orders with the same OMS order number. If a duplicate is detected, the trigger can prevent the new record from being inserted (e.g., by calling `addError()` on the incoming record), thus preventing the creation of a duplicate.