How to add field to validate duplicates when doing duplicate checks?

Hello, 

Does anyone know if there is a way to add a custom field to a duplicate check? Specifically, duplicate accounts. It already checks on name but can we have it check on a custom field, in this case, SAP customer number? I am having data imported through API and it is giving us an error because Sugar is showing them as duplicates. If we could check by the custom field also it should eliminate that issue because they should not match on that item. 

Thank you for your help, 

Larisa Webb