src/Http/HttpHeaders.php
public function add($name, $values, bool $append = false): void
    {
        $normalizedName = self::normalizeHeaderName($name);

        if (!$append || !$this->containsKey($normalizedName)) {
            parent::add($normalizedName, (array)$values);
        } else {
            $currentValues = [];
            $this->tryGet($normalizedName, $currentValues);
            parent::add($normalizedName, array_merge($currentValues, (array)$values));
        }
    }
public function add($name, $values, bool $append = false): void
    {
        $normalizedName = self::normalizeHeaderName($name);

        if (!$append || !$this->containsKey($normalizedName)) {
            parent::add($normalizedName, (array)$values);
        } else {
            $currentValues = [];
            $this->tryGet($normalizedName, $currentValues);
            parent::add($normalizedName, array_merge($currentValues, (array)$values));
        }
    }
Headers are allowed to have multiple values, so we must add support for that

@inheritdoc
@param string|array $values The value or values
@param bool $append Whether or not to append the value to to the other header values