src/Composer/DependencyResolver/Problem.php
protected function addReason($id, $reason)
    {
        if (!isset($this->reasonSeen[$id])) {
            $this->reasonSeen[$id] = true;
            $this->reasons[$this->section][] = $reason;
        }
    }
protected function addReason($id, $reason)
    {
        if (!isset($this->reasonSeen[$id])) {
            $this->reasonSeen[$id] = true;
            $this->reasons[$this->section][] = $reason;
        }
    }
Store a reason descriptor but ignore duplicates

@param string $id     A canonical identifier for the reason
@param string $reason The reason descriptor