The rules will change both for those who already are Norwegian citizens, and for those who are applying for Norwegian citizenship.
In order for you to have dual citizenship, the other country you are a citizen of must also allow this.
You must always check whether the country you are a citizen of accepts that you get dual citizenship, or if you will lose your citizenship automatically if you become a Norwegian citizen. You must check this with the authorities in this country, for example through an embassy.
The Directorate of Immigration (UDI) is the central agency in the Norwegian immigration administration, and can answer questions about the citizenship rules. Here is the contact info for the UDI.
For more information plese visit the UDI website.