Random draft invoices + incorrect due date

I have random Draft invoices appearing. They're clones from old invoices that are months old.
Today I have an invoice that was sent out, but it's showing as a draft and due in a week, but when I edit it, it's actually due in two weeks.

Has anyone else experienced something similar in invoicing?

I don't have such an issue.

Thanks. I also noticed that my balance is out compare to my bank's, but thousands.
Upon further investigation it would appear as if my client, when they pay multiple invoices in a single transaction, an invoice with that amount is created rather than the existing ones linked to the amount, thus duplicating the amounts. I still haven't gotten to the bottom of it, so can't work out what I am doing wrong.