I presume that when you are talking about Duty Drawback then it means that you are availing All Industry Duty Drawback on the exports made by you. In the All Industry Duty Drawback it is assumed that inputs used in the manufacturing of finished goods for export purpose must have incurred the customs duty at any stage of manufacturing, therefore, it is given back to the exporter on wheigted average method and standards have been fixed accordingly.
Duty Drawback and availing the CENVAT credit of Input Services are two different things. You may avail the both benefits simultenously.
Please also check it with your counsel or with your Range Superintendent.