Hello and welcome Kekay.
There is is a subforum dedicated to
Kindle as well as a sub forum for
Kindle Developers where people may be more 'in the know' regarding these things.
ilovejedd summed it up well. Duokan is no more, hasn't been a thing for some time. There is no way to install another OS that I am aware of, but the good folks on the Developer's Forum have created a jailbreak and using it you can definitely gain some additional functionality
Good luck!
