mistakenly allotted by gst registration by twice

our one of the customer applied gst registration due to some mismatch problem registration applying 25 days overed
he applied again with another email and cell number. and taken gst registration certificate. but previous applied gst application also approved and issued another gst registration.. but our customer don't that.. today he was rec'd one notice from Dept for not filling of gst returns for previous registration. he don't know about the second registration. what can he do for cancellation.