Before working a new Lead in Salesforce.com, use the “Find Duplicates” button, located in the Additional Information section of a Lead, to search all Leads, Contacts, Accounts, or Opportunities for duplicate information.
By default Salesforce will search every field you have populated in the Lead record. When you click on Search, Salesforce will show you ALL Leads, Contacts, Accounts and Opportunities that match any of criteria selected in the Lead.
If a match is found you can use the Convert Lead button to covert the Lead with the appropriate Lead, Contact, Account or Opportunity.
Following this process will prevent you from creating duplicate information as well as from working a lead that may already be in Salesforce!