A and C

CORRECT ANSWER IS -
A: Use the order number from the OMS as an external ID.
C: Ensure that the order number in the OMS is unique.

Explanation Why Correct:
A: Using the order number from the OMS as an external ID in Salesforce allows the integration to check if the order already exists before creating a new one, thus preventing duplicates.
C: Ensuring that the order number in the OMS is unique is crucial because if the order numbers are not unique, even using them as external IDs would not prevent duplicates.

INCORRECT OPTIONS:
B: Using the email on the contact record as an external ID is not a reliable method to prevent duplicate orders because multiple orders can be associated with the same contact email.
D: Writing a before trigger to delete duplicates is not an efficient solution. It is reactive rather than proactive and does not address the root cause of the problem, which is the creation of duplicate records. It is better to prevent duplicates from being created in the first place....

A and C