Duplicate invoices, overdues being sent with $0 balance

hi all,

I am getting some strange things happening with WHMCS. Firstly, I swapped servers and after a week or so everything seemed to settle down and worked fine. But now I am getting more than 1 problem.

1. Duplicate invoices are being sent with same invoice number.
2. Overdue invoices are being sent with $0 amount after the client has already paid.
3. Overdue invoices are being sent after client has paid and showing original amount as unpaid on the invoice but not in the admin section.

Anyone have any ideas as to what might be causing this as its getting very frustrating not only for me, but for my clients.

Thanks in advance

Glenn