I need to implement a verification if the user is already logged in. At the moment my game doesn't verify this, so an account can log-in several times and I want to avoid this.

This is how I'm doing log-in authentication on game server:

void Session::authLogin(uint32_t id, std::string username, std::string token)
{
	// Return a valid alive connection, if doesn't exist, return nullptr
	auto dbcon = DatabaseManager::getFreeConnection();
	
	// Wait for a free connection
	if(!dbcon || dbcon->is_locked())
	{
		dbWait->expires_from_now(
		boost::posix_time::microseconds(
		DatabaseManager::DB_WAIT_TIME));
		
		dbWait->async_wait(boost::bind(
			&awaitAuthLogin, 
			boost::asio::placeholders::error, 
			weak_from_this(), 
			id, username, token));
		
		return;
	}
	
	// Lock connection
	DatabaseManager::Locker lock(dbcon);
	
	// Get Account Data
	auto pstmt = dbcon->prepare_statement(
			"SELECT `username`,`storage_gold`,`coins`,`ctl_code`,`vip_level`,"
			"unix_timestamp(`vip_expire_date`) FROM `account_data` WHERE `id`=? AND `username`=?");
	
	pstmt->push_uint(id);
	pstmt->push_string(username);
	
	uint32_t storage_gold = 0;
	uint32_t coins = 0;
	int32_t ctl_code = 0;
	uint32_t vip_level = 0;
	uint32_t vip_expire_date = 0;
		
	auto result = pstmt->execute_query();

	if(result->next())
	{	
		//Check token
		pstmt = dbcon->prepare_statement(
			"SELECT COUNT(*) FROM `token` WHERE `account_id`=? AND `token`=?");
		
		pstmt->push_uint(id);
		pstmt->push_string(token);
		
		auto token_result = pstmt->execute_query();
		
		if(!token_result->next())
		{
			// Invalid Token
			sendMessage(strdef::INVALID_TOKEN);
			closeSession();
			
			return;
		}
		
		storage_gold = result.get_int("storage_gold"),
		coins = result.get_int("coins"),
		ctl_code = result.get_int("ctl_code"),
		vip_level = result.get_int("vip_level");
		vip_expire_date = result.get_uint("unix_timestamp(`vip_expire_date`)");
	}
	else
	{
		// Invalid Login
		sendMessage(strdef::INVALID_USERNAME);
		closeSession();
		
		return;
	}

	// TODO: Check if account is logged in

	// *** I was thinking of doing an INSERT and 
	// if I get an ER_DUP_ENTRY error  it's because the account is logged in.
	
	try {
		// Insert account_stat
		pstmt = dbcon->prepare_statement(
			"INSERT INTO `account_stat` (`account_id`,`server_id`,`endpoint`,`mac`) VALUES(?,?,?,?)");
			
		pstmt->push_uint(id);
		pstmt->push_uint(getServerId());
		pstmt->push_string(getEndpoint());
		pstmt->push_string(getMacAddress());

		pstmt->execute();
	}
	catch(const mysql::exception& e) {
		// Already logged in
		if(e.error_code() == ER_DUP_ENTRY){
			sendMessage(strdef::ALREADY_LOGGED_IN);
			closeSession();
			
			return;
		}
	}
	
	// ***
	
	authLoginTimeout->cancel();
	
	// Make AccountInfo
	accountInfo = std::make_unique<AccountInfo>(id,
		username,
		token,
		storage_gold,
		coins,
		ctl_code,
		vip_level;
		vip_expire_date);
	
	// Load account storage
	loadStorage(id);
	
	// Send character list
	sendToCharacterLobby(id);
}

Can I have a problem doing this? Is there another way? I need it to be safe for multiple gameservers/dataservers.

If you have any tips to optimize it I accept :-)

Thanks in advance for helping me with this.