How to use a trigger to only insert one row

I am doing a third year project using an intel galileo that reads an RFID card, and signs a student into a class, and I am using multiple tables.

I have nearly completed it as all reads are been recorded into a table called read_log. What I want to achieve is when a record is entered into this read_log, set a trigger to just insert the card number, which again is been sent this table read_log.

So what i have so far is :

DROP TRIGGER IF EXISTS `test`;

CREATE DEFINER=`root`@`localhost` TRIGGER `test` AFTER UPDATE ON `readlog` 
FOR EACH ROW
BEGIN 
INSERT into attendence(S_no) 

SELECT s.Stu_no from student s 
JOIN readLog r ON s.Stu_no=r.Cardid WHERE s.Stu_no=r.Cardid; 

END

I can confirm the trigger is working, as once i scan the rfid card over the galileo it inserts into the database, and the trigger triggers but enters the number 29 times, so there is 29 records entered everytime i read.

Can someone provide a tip as to what to look for or how to go about writing a trigger that will only insert one row. I have tried update instead of insert but this does nothing.