That's Not My Neighbor

Dear 너, we are pleased to welcome you as a new employee of the Doppelganger Detection Department (D.D.D.)
Your new job is simple, but very responsible. You have to check on residents trying to get inside building. If you suspect that the person trying to get inside the building is a Dopelganger, you can chase him away or call D.D.D. if he won't leave. You have to check on five people, after which the working day is over. A D.D.D. agent will come to you after work and provide you with any information or instructions, if necessary.
Your workroom does:
- An archive with data from residents on all three floors.
- Calendar
- A phone number to contact residents or D.D.D. at 3312.
- A shredder for forged documents.
- An alarm button in case something unexpected happens, the button automatically locks the doors and calls the D.D.D. PS. use only in EXTREME cases.
- Door control mechanism. Two buttons, open and lock. PS. Don't forget to close the doors.
A "simple" pass system is in place at this time. A simple pass contains a photo and name of a person. In some cases you may be ordered to pass only with advanced passes. An advanced pass contains a lot of different information, but the specific content depends on the situation in your area. Simple passes are also obligatory in any case, i.e. in case of an order there must be both simple and advanced passes.
You are not allowed to leave the building under any circumstances. Our team hopes you will have a long and happy working life.
PS. If you are not sure about the person, but the appearance and documents are in order, try to get him to talk. Doppelgangers are usually not very smart and have little idea what people are doing.
Best regards D.D.D.

You put the paper aside and mark "day 1" on the calendar, you hear footsteps in the distance, someone is approaching.