When using bulk to upload records, Marketo supports only upsert functionality (createOrUpdate). If you want to only create but not update records, include bulkUploadRecordSize in the metaData passed with the bulk request. Where bulkUploadRecordSize is the number of records (up to 300) in the upload. 

Requests that specify bulkUploadRecordSize can quickly use up API rate limits. See Should I be concerned about rate limits? for more.

Here's an example: