This applies both to those who already are Norwegian citizens, and to those who are applying for Norwegian citizenship.
You must also check the rules of the other country
In order for you to have dual citizenship, the other country you are a citizen of must also allow this.
You must make sure what the rules are in the country you are now a citizen of. You can for example look for information on an official website which belongs to the authorities in this country or ask an embassy.
You do not have to inform Norwegian authorities that you wish to keep your previous citizenship.
Norway allows dual citizenship from 1 January 2020 - UDI