Plugin to prevent Duplication

Hi All,

I need to create a plugin to prevent the duplication but with a different Criteria The scenario :

I have Maintenance department and service department so fro example :

Contract_number           Department

1002                               Maintenance

1002                               Service

This is right

Contract_number           Department

1002                               Maintenance

1002                               Maintenance

This is wrong

So the following code will prevent the duplication for all contract numbers how can i convert it to be for the departments level The Code:

if (contract.Contains("new_name"))              contractNumber = (string)contract.Attributes["new_name"];              else if (contractImg.Contains("new_name"))                  contractNumber = (string)contractImg.Attributes["new_name"];              QueryExpression qExp = new QueryExpression();              EntityCollection eCol = new EntityCollection();              qExp.EntityName = "new_contract";              qExp.ColumnSet = new ColumnSet("new_name");              qExp.Criteria.AddCondition("new_name", Microsoft.Xrm.Sdk.Query.ConditionOperator.Equal,(contractNumber));              eCol = service.RetrieveMultiple(qExp);                           if (eCol.Entities.Count > 1)              {                throw new InvalidPluginExecutionException("Contract Number already exists ");                        }