Assigning An Invoice Number To An Order

I am trying to assign an invoice number to orders that are made on my website so that i can combine multiple order numbers into one invoice number. Here is the code i am using to insert the invoice number into the orders table.

It works if there is only one order in the table but if you try and order again it multiplies the orders by the number of invoice rows for example if there was 3 invoice fields it would add the same order 3 times into the order table. Its got stuck in some sort of loop. Code: