Good morning Paul,
I have done some thorough testing on the booking engine, I had the boss's credit card and I booked myself onto several tours and 'unfortunately' didn't come across any errors, the booking engine is a complex system and every now and again can throw up unexpected errors for no apparent reason, if it does occur again, if you could hit your 'print screen' button, then right click on your mouse and 'paste' the screenshot into an email and send it to me at btaylor@justyou.co.uk that would be appreciated 🙂
As for the duplicate invoices, have you had any changes made to your booking? Or have you bought any extras, for instance, excursions, insurance, visa etc? If so, a new invoice will be sent to you on each occasion, not ideal I know and I too feel bad for the environment, I am driving for all this to be taken online so that you the customer can print an invoice off once, but can readily see your account online if you make an amends etc, this of course is an ideal world scenario, I'm sure we'll get there!
I hope this helps?
Kindest,
Ben