Hi
Am getting the this error "System.ListException: Duplicate id in list"
can any one please tell me,how to avoid this error
below is the my code.

global with sharing class Task_Esc_duedate_users_to_crm_user implements Schedulable
{
public List<Client_Requirement__c> clientreq=new list<Client_Requirement__c>();
public list<task> listtask=new List<task>();
public list<task> listtask2=new List<task>();
public set<id> CRID=new set<id>();
public list<user> crmusernewid=new list<user>();


global void execute(SchedulableContext SC) {
try
{
map<Id, User> Id2User = new map<Id, User>([SELECT Id, Name FROM User WHERE Profile.Name IN ('East','Howrah','North and Central','Rajarhat','South I','South II','Newtown')]);
crmusernewid =[SELECT Id, Name FROM User WHERE Active_user__c=false and Profile.Name IN ('System Admin')];

clientreq=[select id,name,RecordTypeid,Ownerid  from Client_Requirement__c];
system.debug('List of all  client requirement'+clientreq);
for(Client_Requirement__c cr: clientreq)
{
CRID.add(cr.id);// fetching the list of all client req IDS
}
listtask=[Select t.CreatedDate, t.Id, t.LastModifiedById, t.LastModifiedDate, t.OwnerId, t.Priority,
t.Status, t.Subject, t.WhatId, t.WhoId,t.ActivityDate,t.Due_Date__c from Task t where Whatid=:CRID and Status='Not Started'  and  ActivityDate=YESTERDAY AND OwnerID IN :ID2User.keyset()];
system.debug('List of tasks that are not closed with in due date of all users '+listtask);
for(task t: listtask)
{
for(user crm:crmusernewid)
{
t.OwnerId=crm.id;// Assigning the crm owner to the all the tasks
listtask2.add(t);
}
}
update listtask2; //updating the tasks
system.debug('@@@@ List of tasks that are updated to user crm head '+listtask2);
}
catch(Exception e){system.debug('@@@@@@@@@@@@@@'+e);}
}
}

Functionality is assiging the all users task to crm head user.after one day.

Regards
venkatesh