for ( List<Task> tasks : [Select Id from Task where Status like 'Completed' 
                          and Subject like 'Delete Me' 
                          and CreatedDate < :(System.now().addDays(0-days+1)) Limit 100]) {
    numDeleted += tasks.size();
    delete tasks;
}

TRIGGER CODE:

trigger ContactDuplicatePreventor on Contact (before insert, before update) {

    Map<String, Contact> ContactMap = new Map<String, Contact>();

    for (Contact contact : System.Trigger.new) {    

        // Make sure we don't treat an email address that
        // isn't changing during an update as a duplicate.

        if ((Contact.Email != null) && (System.Trigger.isInsert || (Contact.Email != System.Trigger.oldMap.get(Contact.Id).Email))) {
    
            // Make sure another new Contact isn't also a duplicate

            if (ContactMap.containsKey(Contact.Email)) {
                String errortext = 'Another new Contact has the same email address. ('+Contact.Email+') Please search for the existing Contact record with this address and update as appropriate, or contact your Administrator for assistance';
                Contact.Email.addError(errorText);
            } else {
                ContactMap.put(Contact.Email, Contact);
            }
        }
    }

    // Using a single database query, find all the Contacts in
    // the database that have the same email address as any
    // of the Contacts being inserted or updated.
 for(String test:ContactMap.keySet()){
 system.debug('\n\nContact added to Map was:'+test);  //only here for testing purposes
 system.debug('\n\nUser running trigger is:'+UserInfo.getUserName());
 system.debug('\n\n'+UserInfo.getProfileId());
 }
    for (Contact contact : [SELECT Email FROM Contact WHERE isDeleted = false and Email != null and Email IN :ContactMap.KeySet()]) {
        SYstem.debug('In the test loop');
        Contact newContact = ContactMap.get(Contact.Email);
  String errortext = 'Another Contact has the same email address. ('+newContact.Email+')  Please search for the existing Contact record with this address and update as appropriate, or contact your Administrator for assistance';
        newContact.Email.addError(errorText);
    }

TEST CASE CODE:
public class testBlockDuplicateContactEmails {
 
 static testMethod void testDuplicateContactEmailTrigger(){  
 
  //Creating all records against "Acme Corporation - HQ" Account (00100000002qK3l)
  Contact[] smith1 = new Contact[]{
   new Contact(  Email='smith@acme.org', LastName='Smith', Accountid='00100000002qK3l' )
  };
  insert smith1;  // add a known Contact
  
  Contact[] smith2 =new Contact[]{
   new Contact(  Email='smith@acme.org', LastName='Smith', Accountid='00100000002qK3l' )
  };
  // try to add a matching lead
  try {
   insert smith2;
   } catch (System.DmlException e) { 
   String errortext1 = 'FIELD_CUSTOM_VALIDATION_EXCEPTION, Another Contact has the same email address';
   system.debug('INSERT ERROR MESSAGE IS:'+e.getMessage());
   system.assert(e.getMessage().contains(errortext1), e.getMessage());
  }
  
  // test duplicates in the same batch for bulk Contact loading or lead conversion
  Contact[] bulkcontacts =new Contact[]{
   new Contact(  Email='Johnson@acme.org', LastName='Johnson', Accountid='00100000002qK3l' ),
   new Contact(  Email='Johnson@acme.org', LastName='Johnson', Accountid='00100000002qK3l' )
  };  
  try {
   System.debug('\n\n Insert Test');
    insert bulkcontacts;
    } catch ( System.DmlException e) { 
   String errortext2 = 'FIELD_CUSTOM_VALIDATION_EXCEPTION, Another new Contact has the same email address';   
   system.assert(e.getMessage().contains(errortext2), e.getMessage());
  }
  
  // test update also
  Contact[] contactup = new Contact[]{
   new Contact(  Email='jones@acme.org',  LastName='Jones', Accountid='00100000002qK3l' )
  };
  System.debug('\n\n Update Test');
  insert contactup;
  Contact updatetest = [ select id,Email from Contact where Email = 'jones@acme.org' and Accountid='00100000002qK3l' limit 1];
  system.assert(updatetest!=null);
  updatetest.Email = 'smith@acme.org'; 
    
  try { update updatetest; } catch ( System.DmlException e) { 
   String errortext3 = 'FIELD_CUSTOM_VALIDATION_EXCEPTION, Another Contact has the same email address';
   system.debug('UPDATE ERROR MESSAGE IS:'+e.getMessage());
   system.assert(e.getMessage().contains(errortext3), e.getMessage()); 
  }
  // Try a null email address
  Contact[] contactnull = new Contact[]{
   new Contact(  LastName='Anderson', Accountid='00100000002qK3l' )
  };
  System.debug('\n\n Null Test');
  try{ insert contactnull;} catch( System.DmlException e){
   system.debug('\n\nNull Insert ERROR MESSAGE IS:'+e.getMessage());
  }
  // Try a null email address Update
  Contact[] contactnull2 = new Contact[]{
   new Contact(  LastName='Anderson', Accountid='00100000002qK3l' )
  };
  System.debug('\n\n Null Update Test');
  insert contactnull2;
  Contact updatetest2 = [ select id,Email from Contact where Email=null and LastName = 'Anderson' and Accountid='00100000002qK3l' limit 1];
  system.assert(updatetest!=null);
  updatetest2.FirstName = 'Hans'; 
    
  try { update updatetest2; } catch ( System.DmlException e) { 
   system.debug('UPDATE ERROR MESSAGE IS:'+e.getMessage());
    
  }
 }
}