An update query which will only insert unique records

I am having trouble creating this, my current problem is that i have a query where a list of invoices is created, i wish to store these invoices in a table called invoice, where it will be given an auto number field. However, if the query is run twice, i dont want the repeated invoices entered again. I was thinking i could use my client_id field and payment_count field as a unique identifier, and have a where clause somtheing along the lines of

WHERE payment.client_ID <> invoice.client_ID AND payment.payment_count <> invoice.payment_count


however i am not sure how to implement this, could anyone help me please?

Check into using the "If Exists..." clause of a sql statement.