Find the values two arrays share

Permission checks, tag filters and "also bought" panels all reduce to the same question: which values appear in both lists?

Implement intersection(a, b). Return a new array holding every value from a that also appears in b, in the order they first appear in a, with duplicates removed so each shared value appears exactly once. Compare values the way a Set does: NaN matches NaN, but 1 does not match "1". If there is no overlap, or either array is empty, return an empty array. Never modify either input.

What it has to do

• Include only values present in both arrays.
• Order the result by first appearance in a.
• Include each shared value only once, even if a repeats it.
• Match NaN with NaN, and keep 1 and "1" distinct.
• Do not mutate either input array.

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