This setting is turned off by default.

Lead Liaison associates a Lead Liaison Prospect to a Salesforce.com Lead or Contact by email address. For select installations, this may introduce new challenges since Salesforce.com allows new Leads/Contacts to be created with any email address even if other Leads/Contacts have the same email address. To work around this, it may be necessary to link a Lead Liaison Prospect to the most recent Salesforce.com Lead/Contact created with the same email address. Lead Liaison provides a "Link to Most Recent SFDC Record" setting which can be turned on and will do two things when enabled:

  1. Renew the association between a Lead Liaison Prospect and a Salesforce.com Lead/Contact with the same email address when a new Lead/Contact is created in Salesforce.com with the same email address. The original association will be removed and a new association will be established to the Salesforce.com with the most recent create date. 
  2. Migrate the "Lead Liaison Marketing Data" and "Lead Liaison" sections (if added to your Salesforce.com Lead/Contact layout) from the original association to the new association as defined in #1 above. 

To access this setting go to Settings > Account > Settings > Salesforce.com and turn the "Link to Most Recent SFDC Record (duplicate emails)" setting on.