When using an Upsert operation for a Marketo entity, you must use an ID. If you do not include an ID, a new record is created, which could result in duplicate records.
If you include an ID, and the ID does not match an existing record, the Upsert fails because Marketo needs to generate an ID for the new record being inserted. Use a Lookup operation first to determine whether or not the ID exists. If the ID exists, use an Upsert operation containing the ID to update the existing record. If the ID does not exist, use an Upsert operation with no mapped ID field to insert the new record.
For additional information, see the following in the TIBCO Scribe® Online Help: