does the 30 days period (on a 30 days double entry visa) restart on the second entry to china?

Yes, the 30 days duration of stay (on a 30 days double entry visa) restarts on the second entry to China. But bear in mind that you have to use the visa to enter China before the "Enter Before Date"

You can
1. entry to china 15. sept 2011
exit to hongkong 5. oct 2011
2. entry to china 10. oct 2011 (restart of the 30 days duration of stay)
exit to thailand 4. nov 2011