This API is part of the Hack Standard Library, not HHVM itself.

Returns a new dict in which each value appears exactly once

namespace HH\Lib\Dict;

function unique<Tk as arraykey, Tv as arraykey>(
    KeyedTraversable<Tk, Tv> $traversable,
): dict<Tk, Tv>;

In case of duplicate values, later keys will overwrite the previous ones.

For non-arraykey values, see Dict\unique_by().