I once cleared customs in HKG because I was on standby on the next flight. So I decided to collect my bags. BINGO. I got on but the flight didn't leave for 12 hours! So I went through security to The Wing to shower and leave my bag with the FC reception.
Of course then I wanted to leave the secure area and re-enter HKG. Well this was not easy because they I had already entered HKG an hour earlier.
After a 20 minute delay a customs and a STERN talking to, they let me re-enter HKG.
BOTTOM LINE: Only enter HKG once per day!