Commerce PayPal commerce_payment_transaction PDOException[23000]
Duplicate entry '' for key 'name': INSERT INTO {users} (uid, created, data)


This is incorrect code, since no name value defined for inserting, so user name is empty and name should be unique - you cann't insert second user with empty name.

Try find this code INSERT INTO {users} (uid, created, data) on your drupal files (modules), if no success, investigate your Rules for custom code.