Different countries have different tax laws. If you are resident in Portugal and receive income from another country, you will not be double-taxed over the same income in both countries. This is known to be the “Double Taxation Agreement” between borders.
To find more about the current countries that have an agreement with Portugal, please click on the link below:
Relações Internacionais (portaldasfinancas.gov.pt)
For countries that have no double taxation agreement with Portugal and are not Black Listed, the OCDE Double Taxation model is applicable to avoid the Double Taxation.