It really depends on the two countries involved, and which one is your original citizenship. For example, I was born in the US, so originally a US citizen. I subsequently became a citizen of Australia, but was allowed to maintain my US citizenship also. But if I was born in Australia and wanted to become a US citizen, I believe I would have had to give up my Australian citizenship. It may be similar with HK - if you are born here you can become a dual citizen with another country, but not the other way around.