It is allowed to have one or more citizenships in addition to a Norwegian citizenship.
It is also possible to get your Norwegian citizenship back, if you have lost it due to the old rules.
You can register your application to get you Norwegian citizenship back in SpN (The Norwegian Directorate of Immigration’s application portal). It is still not possible to book an appointment to deliver the application and supporting documents at the Norwegian Embassy. As soon as the technical solution for it is in place, we will open for booking of appointments.
You will find more information at UDI