When surveilling a room, each guard post in a room is only meant to bring in one guard. If there's one guard post in a room and you roll two surveillance dice and both get a guard result, you ignore the second result and place one guard.
On p. 39 of the Rules Reference, it says: "If you are instructed to place a guard on a space that is already occupied, place the guard on the closest unoccupied space of your choice."
This is not meant to instruct you to place multiple guards from the same post. The intention of this text is to account for situations where a bot, different guard, special equipment, or other component occupies the guard post. Something else being on the guard post doesn't prevent a guard from being placed there. The guard is simply on the closest space it can occupy.