The most common way an individual gains citizenship is by birth. If the country of birth permits second citizenship, you can obtain it through several different ways.
In most cases, the first citizenship comes from the country in which you or your parents were born; this is referred to as birth citizenship. It can also be the case that you have obtained citizenship by naturalization, after living in that country. Alternatively, you can gain citizenship by investing in a country of your choice.
Holding citizenship in two or more countries is referred to as dual citizenship. Holders have legal obligations and rights in all countries where they are citizens. The legislation of some countries allows for dual citizenship, while other nations have created laws against it.
As an example: China, United Arab Emirates, Saudi Arabia, and Andorra, among others, forbid dual citizenship. Still, some of these countries may offer exemptions, as it is the case with Azerbaijan, where the President may offer dual citizenship to people of special importance.
Japan, for example, gives a Japanese citizen until the age of 22 to decide their country of loyalty if they have obtained dual citizenship through a country that allows it.
Although dual citizenship is not permitted in Singapore, there are also exceptions. Similar to Japan, a child born abroad to Singaporean parents, may maintain dual citizenship until the age of 21 years. After 21, the individual has 12 months to take the Oath of Allegiance and Loyalty and renounce the foreign citizenship, or their Singaporean citizenship will be revoked.