Without being able to reproduce the problem, it is impossible to troubleshoot the cause.
Start by adding the error reporting code, and adding a message next to the submit order button to warn customers not click the button a second time if not redirected.
if they are not redirected, the error reporting code should show a php error, it should also be recorded in the servers error log. If you know the day and time of some of the duplicate orders, you could contact the host to see if they can give you access to the server error log to see what is going wrong. This could give some information to work from.