I want to mention my personal essay in a supplemental essay for why I want to go to "blank" university. is that something I should stay away from?
So I wouldnt name the essay as you don't control what order the admission will read but if your personal essay is about determination you could say My determination caused me to be involved in blah blah blah. So you can bring that topic into the essay but don't have it be a copy n paste or rehash info.
HTH!
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I wouldn't do that just because you want the admissions committee to see as many sides of you as possible from just a few essays, so repeating content is a waste of an opportunity to show them something else about you.