How do I duplicate a RFID keyless entry fob?

Sorry, going to management is the only option. RFID tags have unique serial numbers, which the receivers are trained to recognize, the processes is often called "learning". You never really "duplicate" an RFID tag, you simply "learn" a new one.

You don’t, at least for that purpose. You need to get another one from building management. If everything is on the up and up, you won’t have a problem.

You obviously do NOT understand the technology of RFID’s. It is the whole POINT that you do not have the ability to copy them that makes them a security device! There are two different types of technology in use, resonant circuits and active circuits. Resonant circuits are "tuned" strips of metal, When placed in the field from the detactor, they resonate which affects the field in a particular unique manner. If that unique resonance is on the list of those allowed entry, the lock clicks, otherwise nothing happens, or the lock could be programmed to trigger an alarm because the key was stolen. The things that the lock can be programmed to do are open to whatever you want. The active circuit type contains a smart chip. Unlike the resonance field, this field works to provide the chip with power much like a battery, which then triggers the chip and transmits a coded signal, again totally unique. Likewise, you will not be duplicating the active key without duplicating the circuitry built in to the key. Neither type can be duplicated without some very special equipment which the typical consumer, such as you, do not have access to. The whole point of RFID security keys IS the security of NOT being able to duplicate them as you can with a simple metal key blank. Why would you have a need to bypass the management? If you are doing everything above board, all you need do is ask for another key, right? I have one of these RFID keys for a storage unit. All I need do to get a 2nd key is pay the key deposit! It seems to me you are thinking of an illegal sublet contrary to the terms of your lease so you get the extra cash instead of your landlord… or maybe this is for a significant other you want to move in which would also constitute an illegal sublet…