Issue with sending invoice emails

Not sure if anybody else has this issue.

I was sending invoices out this morning and noticed quite a few old invoices in the emails that hadn't been sent but i was sure they had been. Unfortunately i didn't check them and just changed the dates to today and re emailed. I then got copious phone calls from customers saying they had already paid or received the email with a different dat on it.

I then went back into each customer and a duplicate invoice had been created as still open.

Why is this happening?

I also noticed that some of my customers that i had changed to 30 days trade customer had been reverted back to their original entry of 7 day customers. 

this happens so regularly it's like MYOB is backing up from old dates.

Sort it out MYOB please it's becoming so frustrating. We have enough stuff to deal with without not having faith in this system.