trigger LeadPreventDuplicate on Lead (after insert) {

    Map<String, Lead> leadMap = new Map<String, Lead>();

    for (Lead lead :[SELECT Id, Email FROM Lead WHERE Id = :Trigger.new]) 
    { 
        if ((lead.Email != null) && (System.Trigger.isInsert || (lead.Email != System.Trigger.oldMap.get(lead.Id).Email))) 
        {
            leadMap.put(lead.Email, lead);
        }
    }
    
    if(leadMap.isEmpty()) return;

    //For Update
    List<Lead> leadToUpdateList = new List<Lead>();  
    Map<String, Integer> emailCountMap = new Map<String, Integer>();

    AggregateResult[] groupedResults = [SELECT Email, Count(Id) FROM Lead WHERE Email IN :leadMap.keySet() GROUP BY Email];
    for (AggregateResult ar : groupedResults)  
   {   
        
        String email = String.valueOf(ar.get('Email'));
        Integer count = Integer.valueOf(ar.get('expr0'));
        
        emailCountMap.put(email , count );
        
        if(count == 1) continue;
      
        //For Update
        Lead leadToUpdate = new Lead(Id = leadMap.get(email).Id);
        leadToUpdate.Duplicate_Lead__c= true;

        leadToUpdateList.add(leadToUpdate);
    }       

    //For Update
    Database.update(leadToUpdateList);
                             
    List<task> addtask=New List<task>();
    
    //For Update
    for (Lead lead : [SELECT Email FROM Lead WHERE (Duplicate_Lead__c= false OR Duplicate_Lead__c = null) AND Email IN :leadMap.KeySet()]) 
    
    {

        if(emailCountMap.get(lead.Email) == 1) continue;

        Lead newLead = leadMap.get(lead.Email);
        addtask.add(new Task(
            WhoID =lead.id, 
            Status = 'Active',
            Subject = 'Test Existing Task',
            ActivityDate = system.today()));
        
    } 
    insert addtask; 

 }

@isTest
public class LeadPreventDuplicate_Test {

    static testMethod void  single_Lead_must_not_create_activity_test(){

		insert new Lead ( LastName = 'Test', Email = 'test@test.com', Company = 'Test');
        
        Lead lead = [SELECT Id, Duplicate_Lead__c FROM Lead LIMIT 1];
        
        List<Task> taskList = [SELECT Id FROM Task];
        
        system.assert(lead.Duplicate_Lead__c == false);
        
        system.assert(taskList.size() == 0);

    }    
    
    static testMethod void  duplicate_test(){
        
        List<Lead> leadToInsertList = new List<Lead> {
            new Lead ( LastName = 'Test', Email = 'test@test.com', Company = 'Test'),
			new Lead ( LastName = 'Test', Email = 'test@test.com', Company = 'Test')
        };
            
        Database.insert(leadToInsertList);

        List<Lead> leadList = [SELECT Duplicate_Lead__c FROM Lead WHERE Id = :leadToInsertList AND Duplicate_Lead__c = false];
        
        List<Task> taskList = [SELECT Id FROM Task];

        system.assert(taskList.size() == 1);        
        
    }
}