Salesforce Lead Duplication

1. trigger leadDuplicatePreventer on Lead
2.                                (before insert, before update) {
3.  
4.     Map<String, Lead> leadMap = new Map<String, Lead>();
5.     for (Lead lead : System.Trigger.new) {
6.                
7.         // Make sure we don't treat an email address that  
8.    
9.         // isn't changing during an update as a duplicate.  
10.    
11.         if ((lead.Email != null) &&
12.                 (System.Trigger.isInsert ||
13.                 (lead.Email !=
14.                     System.Trigger.oldMap.get(lead.Id).Email))) {
15.                
16.             // Make sure another new lead isn't also a duplicate  
17.    
18.             if (leadMap.containsKey(lead.Email)) {
19.                 lead.Email.addError('Another new lead has the '
20.                                     + 'same email address.');
21.             } else {
22.                 leadMap.put(lead.Email, lead);
23.             }
24.        }
25.     }