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.

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