Spanish citizenship whit a dual nationality
Spanish citizenship is based mainly on a principle known as Ius sanguinis (blood right). It is a legal criterion that can adopt an order for the granting of nationality. According to the Ius sanguinis, a person can acquire the nationality of their ascendants by the simple fact of their filiation (whether biological or even adoptive), regardless of whether the place of birth is another country, thereby achieving the dual nationality.
Important factors to acquire Spanish nationality
As Spain is a member of the European Union, Spanish citizens (including those with dual citizenship) can travel and reside freely in any of the countries that are part of the EU.
Spain allows dual citizenship in limited circumstances. It is allowed for all Spanish citizens by origin, provided they declare their will to preserve Spanish nationality at least in the following three years after acquiring another nationality.
In this case, this requirement does not apply to those natural citizens of an Ibero-American country, Andorra, the Philippines, Equatorial Guinea or Portugal and any other country with which Spain maintains a bilateral agreement.
People who seek to have Spanish citizenship as their second citizenship, depends on their country of origin above all. A US citizen can acquire Spanish citizenship and continue to be considered a US citizen for the United States. On the other hand, in this case and in Spain, the person is required to renounce his / her citizenship.
In this case, Spain will consider the person only as a Spanish citizen. The United States will consider that person as a dual citizen, US and Spanish citizen. It is somewhat complex, for that reason, whenever there is any doubt, the most advisable thing is to have a good team of specialized professionals so that they can advise on everything related to dual citizenship.