When a new lead is created the system automatically checks for duplicate leads by comparing lead fields that have been enabled for duplicate checks.
For example, when a new lead is created with the same DBA name as an existing lead, a banner will be shown on the right side to indicate that duplicate leads exist:
You can click on the banner to see a list of duplicate leads and take appropriate action:
To enable a lead field for duplicate checks navigate to Manage > Administration > Leads > Manage Lead fields and click the Add/Remove Fields button on the appropriate lead tab:
After that click Edit Field on the appropriate field, click Options, and select the Enable Dupecheck checkbox:
When you're done click Save on all popups and the new field will now be used for duplicate checks on all new leads.
You can enable duplicate checks for one or more lead fields as needed.