As it says on the tin. The first stamp seems to seal the deal even though you can question and examine.
If you stamp a passport with both stamps, the red stamp always takes precedence and the person has to leave. This means that if you accidentally approve someone but still have their passport, you can reject them. There is no penalty for this.
Once you have given the stamped passport to the entrant, it is impossible to ask for it back, even if you later find an error.