Read and understood your history and concerns.
Penis getting super hard- you must be lucky and very healthy and the local anatomy perfect. And after you masturbate the penis gets even harder and stays erected for another 20-0 minutes this indicates that either you are not satisfied and want more of pleasure or the valves are not getting released.
This is called Priapism, Consult your Urologist and discuss further after clinical evaluation by them.
Hope this helps you.
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