The Chinese Nationality Law makes it difficult but not impossible to be a dual national. The easiest way is by birth. If one parent of the child is a Chinese citizen and the child is born in China, then the child is a Chinese citizen regardless of the other country's nationality laws. This is very useful if your family travels in and out of mainland China a lot as mine does. Saves a lot on visa fees.
As for naturalising, the law says you can't retain your foreign nationality. It doesn't say anything about resuming it though...
And HK immigration only recognise the loss of Chinese citizenship under Article 9 if it is declared to them officially and a fee paid.