candidate update REST

[nishanth]

im getting response for candidate update
{"firstName":"testuser","lastName":"test","username":"rt2r","password":"********","email":"user25@jourrapide.com"}{ "errorMessage" : "error persisting an entity of type: Candidate", "errors" : [ { "propertyName" : null, "severity" : "ERROR", "type" : "DUPLICATE_VALUE" } ], "entityName" : "Candidate" }
WHAT MAY BE THE WRONG