Ashkenazi nussach ends the weekday shacharit service with two kaddish yatoms: one after "Aleinu" and another after the psalm for the day (and seasonal insertions if applicable). Ismar Elbogen's Jewish Liturgy: A Comprehensive History is silent on it. Why is this done? Specifically, why is kaddish yatom said twice?
Aruch HaShulchan 55:4 says that the reason there are 2 Kaddish Yasoms at the end of Shacharis is due to the many Yesomim. (I would presume that only one person said each Kaddish, and this way more Yesomim were able to say a Kaddish at the Tefila).
DoubleAA added in a comment: Recall at the time there were only 2-3 Kaddishes available for mourners per day (after Alenu at Shacharit and Arvit, and in some communities one before Pesukei DiZimra), so allowing one more per day would have a big effect (33%-50% increase).