'Synchronize Users to Contacts' -Fails and gives DB error first time

When We run "Synchronize Users to Contacts" in admin section , we get db error
Error Details:

Array
(
[callback] => Array
(
[0] => CRM_Core_Error
[1] => handle
)

[code] => -5
[message] => DB Error: already exists
[mode] => 16
[debug_info] => INSERT INTO civicrm_uf_match (uf_id , contact_id , domain_id ) VALUES ( 5 , 102 , 1 ) [nativecode=1062 ** Duplicate entry '102' for key 4]
[type] => DB_Error
[user_info] => INSERT INTO civicrm_uf_match (uf_id , contact_id , domain_id ) VALUES ( 5 , 102 , 1 ) [nativecode=1062 ** Duplicate entry '102' for key 4]
[to_string] => [db_error: message="DB Error: already exists" code=-5 mode=callback callback=CRM_Core_Error::handle prefix="" info="INSERT INTO civicrm_uf_match (uf_id , contact_id , domain_id ) VALUES ( 5 , 102 , 1 ) [nativecode=1062 ** Duplicate entry '102' for key 4]"]
)

Also if check users in Drupal "Users" section; and then we run synchronize, It runs fine. But not synchronize all contacts properly.