contact c1 = new contact(id='003000000089012', firstname='1', lastname='2', phone=null);
contact c2 = new contact(id='003000000089012', firstname='1', lastname='2');
contact c3 = new contact(id='003000000089012', firstname='1', lastname='2', phone='');
contact c4 = new contact(id='003000000089012', firstname='1', lastname='3', phone='');
contact c5 = new contact(id='003000000089012', firstname='1', lastname='3');

set<contact> contacts = new set<contact>();
contacts.add(c1);
contacts.add(c2);
contacts.add(c3);
contacts.add(c4);
contacts.add(c5);
for(contact c : contacts){
 system.debug('5 Items:  '+c);
}
 system.debug('5 Items BREAK');

c1.phone='';

for(contact c : contacts){
 system.debug('Same :  '+c);
}
 system.debug('Same BREAK');

contacts.add(c1);

for(contact c : contacts){
 system.debug('c1 Added:  '+c);
}

 system.debug('c1 Added BREAK');

contact c6 = new contact(id='003000000089012', firstname='1', lastname='2', phone='');
contacts.add(c6);

for(contact c : contacts){
 system.debug('New Same:  '+c);
}
 system.debug('New Same BREAK');

contacts.addall(contacts);

for(contact c : contacts){
 system.debug('Addall:  '+c);
}
 system.debug('Addall BREAK');

set<contact>  cnts2 = new set<contact>();
cnts2 = contacts.clone();

for(contact c : cnts2){
 system.debug('Clone:  '+c);
}
 system.debug('Clone BREAK');

set<contact>  cnts3 = new set<contact>();
cnts3.addall(contacts);

for(contact c : cnts3){
 system.debug('New Addall:  '+c);
}
 system.debug('New Addall BREAK');