public function duplicates($callback = null, $strict = false)
    {
        $items = $this->map($this->valueRetriever($callback));

        $uniqueItems = $items->unique(null, $strict);

        $compare = $this->duplicateComparator($strict);

        $duplicates = new static;

        foreach ($items as $key => $value) {
            if ($uniqueItems->isNotEmpty() && $compare($value, $uniqueItems->first())) {
                $uniqueItems->shift();
            } else {
                $duplicates[$key] = $value;
            }
        }

        return $duplicates;
    }