//

When do you find the repetition?

 -3 10 9 -7 -3 9 10 -7 -8 -8 9 1 10 2 -3
             | | |   |     |
            -3 9 10 -7    -8

Testing for -3 10 9 -7 -3 9 10 -7 -8 -8 9 1 10 2 -3

It should work for random inputs too: [-3, 10, 9, -7, -8] should equal [-3, 9, 10, -7, -8]

But what's wrong with this output?
The first number that repeats is -3, then -10 then 9 and so on.

No, read the instructions again, about the first test:

$this->assertEquals(3, count(duplicates([1, 2, 4, 4, 3, 3, 1, 5, 3, '5'])));

The first number that repeats, parsing from the left is 4, then 3 and lastly 1, so the expected answer is [4, 3, 1] it has nothing to do with descending order.

I have no idea when/why that happened... Thanks for fixing :D

Restored previous description.

@A.Partridge why did you change it in the first place?

Thanks :) I think it counts as a one-liner solution

Recommended why? Was there a specific function that offers a better/different solution? If so, happy to hear it.

SPOILER FLAG, DAMN YOU! X(

it's not as if I already told you twice!

spoiler flag, bis

spoiler flag...