How do to the Service Cloud APIs handle duplicate contact records?
Managed Framework APIs (Connect for PHP, Connect REST, Connect Web Service for SOAP)
When email address sharing is disabled (the default), contacts created with duplicate email addresses are inserted into the database with '.0001' appended to the contact's duplciate email address(es). This will repeat with each additional contact created with the same address, with the number in the extension incremented in each case ('.0002' for example).
For further details see