public function multiple(): self
    {
        $unique_array = array_unique($this->getArrayCopy());
        return new ArrayMap(array_diff_assoc($this->getArrayCopy(), $unique_array));
    }
public function multiple(): self
    {
        $unique_array = array_unique($this->getArrayCopy());
        return new ArrayMap(array_diff_assoc($this->getArrayCopy(), $unique_array));
    }
public function multiple ( ) : self { $ unique_array = array_unique ( $ this -> getArrayCopy ( ) ) ; return new ArrayMap ( array_diff_assoc ( $ this -> getArrayCopy ( ) , $ unique_array ) ) ; }
Get duplicate values in an array
@return $this
Get duplicate values in an array