in MS Access 2010 prevent Append query from creating duplicate entries

Dear Experts

I am trying to create  an attendance form for my student database. I found a tutorial here:
https://www.youtube.com/watch?v=DbBky5mNGE4

...and while it works great the first time I run the append query. If a student arrives late and I click on the students [present] check box on the tabular form and run the append query again I get the studenti name, current date, and present (yes/no field) posted again in the table. If I run the query again then each record is in the table three times, etc.

I do I only update the "present" check field on the student who previously did not have a check and leave other students alone.

I tried creating an index composite as described below:

http://en.allexperts.com/q/Using-MS-Access-1440/Creating-Unique-Value-check.htm

but that just prevent any updates at all.

Any suggestions?

Thanks!