Cloned MTN number not resolved

My sister has a MTN cell phone number since 1994 and now suddenly it is shared with somebody else. Is it cloned? How is it possible for one number to have two users? The other user is using all the data and even answer calls from and sending messages to persons on her contact lists. After numerous visits to MTN shops this very inconvenient challenge is still not sorted and continue. MTN should please investigate and get back to the number ********** (NK Lohann - the original holder of the number) with an urgent and final resolution.