Must I pay 15 percent tax in Norway in addition to the tax I pay in my country of residence?

No, Norway has signed tax treaties with many countries. The tax treaties contain provisions that ensure you will not pay tax on the same income in both Norway and your country of residence. Which country you pay tax to varies. This follows from the individual tax treaty. 

When the pension/disability benefits is taxable in Norway, it is the country of residence that is responsible for ensuring that double taxation does not take place. This can be ensured by either not taxing the Norwegian pension/disability benefits or by granting a deduction for the Norwegian tax in connection with taxation in the resident country. Contact the tax authorities where you live for more information.