src/core/Headers.php
public function add(string $header): self
    {
        foreach ($this->getAll() as $tmp) {
            if ($tmp === $header) {
                throw new Exception("The '{$header}' header has already been added.");
            }
        }
        $this->headerList[] = $header;
        return self::$instance;
    }
public function add(string $header): self
    {
        foreach ($this->getAll() as $tmp) {
            if ($tmp === $header) {
                throw new Exception("The '{$header}' header has already been added.");
            }
        }
        $this->headerList[] = $header;
        return self::$instance;
    }
It adds the header to the stack.
@param string $header The page header.
@return self.
@throws \nadir2\core\Exception It's thrown if the passed header has already
been added earlier.