Just Ask Is Singapore allow dual Nationality ?

Singapore does not recognize dual nationality beyond the age of 21, and it strictly enforces universal national service (NS) for all male citizens and permanent residents. Male U.S. citizens who automatically acquired Singaporean citizenship are liable for Singapore national service (NS) once they reach the age of 18. Travel abroad of Singaporean males may require Singapore Government approval as they approach national service age and may be restricted when they reach sixteen-and-a-half years of age. Under Singaporean law, an individual who acquires Singaporean citizenship at birth retains that status even after acquiring the citizenship of another country, including U.S. citizenship.

Males may renounce Singaporean citizenship only after having completed at least two years of national service.
Nationality and International Law in Chinese Perspective
Nationality and International Law in Asian Perspective