When saying Kaddish the Chazzan must be vigilant to wait for the tzibbur to answer Amein before continuing the next words of Kaddish. Shulchan Aruch w/Mishnah Brurah 124:9 MB37
However, at “Amein Yehey Shemai Rabbah…” once the tzibbur has said the word Amein it is not a problem for the chazzan to continue because the tzibbur is not relying on the chazzan to be yotze the words “Yehey Shemai Rabbah….” inasmuch as they are saying these words themselves. Halichos Shlomo citing R’ Shloma Zalman Auerbach 6:17, Piskei Tshuvos 124:15 fn:152
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