There are two parts to the solution to this challenge:
The first part is easily solved if DB2-entity has some auto-increment attribute that you can use to continue where you left off the last time. A creationdate-attribute might do as well, stating in your SQL "WHERE creationdate-attributename > "yourdatevalueforthisrun””
The second part is most easily done by creating an import-mapping, set AccountNumber as key, and create on import:
Any import-objects that already exist will not lead to duplicates in your database.