Coming from another country multiple times

Jaclyn, there is no limit on how many times one can use the 72 hour free transit. As long as you meet the requirement of 72 hour free transit, you can use it for the transit.
Thus, you can go to Chengdu 72 hours and 2 weeks later go to Shanghai for 72 hours. But you should make sure that after transiting in a Chinese city, the next country you fly to is different from the last country you departed.

So you cannot go Japan - chendgdu - Japan or Japan - Shanghai - Japan ?
You would have to go to another city after such as Japan - chendgdu - Seoul ?
Please clarify.

Yes, you cannot travel like this: Japan - chendgdu - Japan or Japan - Shanghai – Japan.
You should travel like Japan - chendgdu – Seoul – shanghai –Japan.