I have been told the customs stamps the passports of transit passengers

I dont need trasit visa for my Beijing-Auckland direct flight, I am not going to leave the airport and it is less than 24 hours transit. But I have a SINGLE entry transit visa for Auckland-Guangzhou-Bejing flights and I wonder is that stamp is ok and is my transit visa for the way back from Aickland is still valid with that stamp in the passport