If a Chinese residence has already paid IIT outside China, should he/she pay IIT in China?

That depends on how much he/she has paid overseas.

The individual income tax on overseas income should be calculated separately for each different country or region and for each different income category according to Chinese law. If the amount paid overseas is less than the amount that should be paid in China, the difference should be paid in China. Otherwise there is no need to pay IIT in China. The portion exceeding China tax can be deducted during the following 5 years.