Whether you, a US Expat, have to pay social security to the country you live in and the US depends on whether the country you reside in has a totalization agreement with the United States.
Generally, if you receive employment income from an American employer you are subject to pay into the US Social Security system. Many affiliates of American companies are also subject to this payment liability.
Dual social tax is a normal tax for a expat to receive. This is because most countries impose social taxes on individuals performing services within their territory.
If the country has a totalization agreement with the United States US expats are not required to pay two social security taxes. Under the US totalization agreement, workers who are eligible to coverage under both the U.S. and a foreign social system are subject to only the coverage laws of the country where he or she is working.