Which is a common error to be aware of when using upsert?

Cloud Kicks often uses Data Loader to upsert Contact records into Salesforce to avoid creating duplicate Contacts.

Which is a common error to be aware of when using upsert?
A . Errors with records being updated and inserted in the same CSV file.
B . Errors with using the wrong external Id will cause the load to fail.
C . Errors when a duplicate Contact name is found cause upsert to fail.
D . Errors with duplicate external Id values within the same CSV file.

Answer: D