Use of forms and inserting new records

After getting help from various people on the forum I am in need of some more guidance on priciples regarding the use of forms and submitting data to the database.

I am adding records one at a time through a simple form which just accepts three items of data.

Q. I currently check to see if the record exists based upon two of the data items before inserting it. Is it best to do this or set a primary key on the two fields and then checking for errors

If the record is created then I want to take the user back to the original form for further input but also present them with a confirmation message.

If the record is not created because it exists already then I want to ask the user if they want to overwrite the existing record and do so if they respond “yes”. Then they go back to the original page.

Q. What is the best way to flow between pages.

Thanks for any help you can give.