@nunyab 

I have seen this before on other devices also, its nothing to worry about.   In short each device on that list that is duplicated has connected to the 2.4 and 5ghz at some stage, therefore its got 2 records (one for each frequency).  This tended to be a common thing in some hardware that had two physical different chips for the seperate frequencies, havent seen it as much in newer hardware though as they tend to have one 'dual frequency' chip

In short, I wouldnt be concerned

In answer to updating the router, the firmware for networking devices cannot be initiated automatically from clients so its probably a red herring.   TP Link I know can only be updated from the Web GUI Interface on the router and this has to be manually started.     What changes did you find you MAC applying?   Its more than likely just uPNP or something like that.    Client devices can open ports using a process called uPNP, this allows it to configure the firewall for gaming / video etc...