Contact's attribute 'is_opt_out' is reset when import with updating duplicate

when user click opt-out link, the contact's 'is_opt_out' attribute is changed to 1(true), but when i import email csv file, and update duplicate email, after that ,the is_opt_out is reset to 0(false) which is horrible to me,

i want to modify the import function, it may be CRM_Import_Parser_Contact::import which has 575 lines code;

oh my god, it is horrible, can you help me?