Why some phones are banned in Pakistan while others aren't
GSM Association IMEI Allocation and Approval Guidelines
GSM Association have the following policy regarding Dual (or Multi) SIM phones:
If each SIM is associated with its own transceiver, then each transceiver/SIM should have its own associated IMEI number. i.e., a dual SIM phone should have 2 IMEI numbers; a three SIM phone should have 3 IMEI numbers etc.
Xiaomi uses same imei for its Chinese phones (different in India due to law there) so it is not in gsma list of approved equipment and PTA wont allow it to be imported. Ask sellers before importing any phone to Pakistan about same or different imei for dual sim and tri sim phones.