This API is part of the Hack Standard Library, not HHVM itself.
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This function is available as Vec\unique() in Facebook's www repository.

Returns a new vec containing each element of the given Traversable exactly once


namespace HH\Lib\Vec;

function unique<Tv as arraykey>(
  Traversable<Tv> $traversable,
): vec<Tv>;

The Traversable must contain arraykey values, and strict equality will be used.

For non-arraykey elements, see Vec\unique_by().