closing as a user error and not an issue within the tests

You are mutating the array array because of a buffer overflow. Note that sizeof do not return the number of elements in an array but the size in bytes of its operand. So an array of 4 ints will have a size of 16 bytes because each int has a size of 4 bytes.

Just fix this minor issue and your code pass :)

Random test cases in C have multiple integers with odd count but description says test cases shouldn't
examples:
for array = {7, 1, 7, 75, 75, 1, -11, -11, 7, 7, 75, -11, -11, -11, -11, 7, 7, 7, 1, 1, 75, 7, -11, 75, 75, 1, 75, 75, -11, 75, 1}
expected 1 but got 75
for array = {59, -34, 59, 59, 59, 59, 59, 59, 59, 59, 59, -34, -34, 59, 59, -34, -34, 59, 59, 59, -34, -34, 59, 59, 59, 59, 59, -36, 59}
expected -36 but got 59

Today I learned something new.

Oh, You are right. Didn't noticed the compatibilty. Sorry.

Reading the link you posted: "Browser compatibility: Node 21".

It is not a bug with the kata, you used API which is not available on Codewars.

There will always be only one integer that appears an odd number of times.

That's not a valid input value.

What to return in case of [1,1,1,2,2,2]?

Fun kata, I liked it

Reminder it's been half an hour, think of the Roman Empire.
Translation Dart

love it - i've solved it in the most inefficient way possible with 2 different arrays and 3 for loops lol welp i'm still learning