If we are talking about the front door for the apartment, it does not make sense to install a second door behind the metal door. It is simply unprofitable. But if the steel door is used as an entrance door from the street, be sure to install it into the outer wall of the house behind wooded, plastic or aluminum door. It will help to prevent condensation and moisture on the inside of the front metal door and will protect the surface from the effects of rain and UV rays.
Please note, that door models metal/metal and metal/door panel are not designed for installation as an entrance door from the street to house.
If the steel door is installed set as a front door from the street to the house, it is advisable to mount visor for protection against heavy rain. Patch on the stainless steel threshold will retain its appearance the whole period of operation of the front door. For homes located in the northern regions, it is necessary to have cold vestibule.