Preventing Overwriting

I'm trying to create an online external form that will update a custom record.

In essence; I'm using a serial number field to link the online form to the custom record to prevent duplication (as they are unique). However, this does not prevent someone from coming along, entering the serial number again (or by entering the serial number wrong) and overwriting existing data in the record.

What I want, is to create a check box called "User Submitted" which will be checked on the first instance someone sends in a form and updates a particular record. In the background, a script will run that uses a search to run through all the records. If the check box is already checked, then it should return an alert or prompt saying that the recorded for that serial number has already been submitted.