void onUpdate(Update* update)
{
    static MutexT lock;
    static hash_set<UpdateID> set;

    ScopedGuard _(lock); //lock here
    hash_set<UpdateID>::iterator it = set.find(update->id);
    if (it == set.end())
        set.insert(update->id);
    else
        return;

    listener->onUpdate(/*some stuff*/);
}

boost::shared_mutex          mutex_;
std::unordered_set<UpdateID> set_;

void onUpdate(const Update& update)
{
    boost::upgrade_lock<boost::shared_mutex> lock(mutex_);

    auto it = set_.find(update.id);
    if(it == set_.end())
    {
        boost::upgrade_to_unique_lock<boost::shared_mutex> unique_lock(mutex_);
        set.insert(update.id);
    }

    listener->onUpdate(/*some stuff*/);
}