Prevent Resubmitting The Same Form More Than Once

I have a form which when submitted will validate the fields and then add the record to a database. If a mandatory field is not completed, there is an instruction to return to the form and complete the mandatory fields and then submit.

The problem I have is preventing the user from resubmitting the same form once it has been added to the database. I cannot deactivate the submit button, because if they need to return to complete mandatory fields, the submit button is still required.

I have read the article "Programming Forms To Submit Only Once" from 15seconds.com and this looks exactly what I need......

To prevent the same form from being submitted more than once we must:

Initialize the data structures.
Create a mechanism to give each form a unique identification.
Find a way to store a list of already submitted forms.
When processing a form submit, we just have to check if its ID is on the list.

I am having problems when adding the recommended code to my form and my process form pages.