2 breakers powering 1 set of outlets
The new breaker isn't unsafe, the old one is.

The old breaker--which I presume is still powering the original circuit--is still providing electricity to a circuit which appears to have a problem hidden somewhere in one of your walls.

To be completely safe, shut off the old breaker (which will, unfortunately, also shut off power to everything else that is on the old circuit).

But if the electrician installed a new breaker and ran new wire to the old outlet box like you seem to be saying (and capped off the old wires in that outlet box), that new breaker and new line are perfectly safe presuming the electrician is competent.