On 6 December 2018, the Storting (Norwegian parliament) adopted a resolution that entitles anyone who is or wishes to become a Norwegian citizen to dual citizenship.
This means that Norwegians will be able to retain their Norwegian citizenship if they become citizens of another country. The same principle will apply to citizens of other countries if they satisfy the conditions for Norwegian citizenship. Those who have lost their Norwegian citizenship under the existing rules will be able to reclaim it.
The new rules will not enter into effect before January 1st 2020. Please check the UDI websites for official information and procedure. Until then, the existing rules regarding dual citizenship still apply.
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.