Question: Is there any error handling for failure to create duplicate leads in Marketo?

Answer: No. When an attempt to create a Lead through either POST /leads or a bulk upload fails due to duplicate field constraints, the response body includes no information to describe the offending record other than a generic provider message. Unfortunately, Marketo does not return the offending Lead ID when duplicates are encountered. Although not explicitly documented by Marketo, the Marketo documentation confirms that the examples of error 1005 ("Lead already exists") does NOT include a record/lead ID in the response and the duplicate records are simply 'skipped'.

Due to this Marketo limitation the /leads resource and bulk upload to the leads object do not provide error handling details in the response body when a failure to create duplicate leads is encountered.