Check for duplicate status callbacks before adding to the list
Check for duplicate status callbacks before adding to the list

Duplicate status callbacks don't bring any value and can cause
memory leaks if caller calls for this function multiple times with
same status_cb. Also remove all the instances of the status_cb when
removing event listener.

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed