Because sentinal values are a bad idea for many reasons, though you probably already learned this in the 4 years since.

However I'm still bamboozeled as to why it returns Option<Vec<i32>> as opposed to Option<[i32; 3]>. This isn't an arbitrary list, it's exactly 3 elements. I might even expect Option<(i32, i32, i32).

Honestly I'm not sure what this kata is intended to teach. It's not programming, it's not the math involved. Obviously. Maybe how to not get upset by misleading requirements. Well that would be something. I myself got very upset, as I missed it a couple of times. This kata was a waste of time and almost got me leaving the platform. Because, that is not what I want to learn here.

Read this agn and agn... return the count of distinct case-insensitive alphabetic characters and numeric digits that occur more than once

Testcase
$this->assertSame(1, duplicateCount("Indivisibility"));
is wrong should be 6