Duplicate Record Error Message?

I have a booking form, called classroomform. On this form the user requests a classroom number, location, date, start/end time and number of students.

As it stands, the data is submitted via a submit button and an insert query created using Dreamweaver.

I already have various validation on the form, such as javascript which error checks for invalid times etc and asp validation which checks for correct dates etc.

However, the final touch that I'm looking to do is to add some code to the form which checks for duplicate entries that are in the database. I basically want an error check which doesn't allow the user to enter a duplicate record that is based on classroom number, location, date, start/end time.

Here's my code: ...