Is the duration of a shadda-marked letter the same as that of other letters? And is it true that people pronounce all letters in the same way, without distinguishing between a shadda-marked letter and others?

A doubled letter consists of two letters (the first is silent and the second is vocalized) that have been merged to become a single letter. The duration of pronouncing a doubled letter cannot be determined except through oral instruction. Prolonging recitation in reading and prayer is considered a form of waswasah (whispering of Shaytan/obsessive thoughts), and its cure is to ignore it and pay no attention to it. Prayer is not invalidated by exaggerating the doubling, but it is preferable to limit it to the known standard for recitation.