Duplicate invoices and cheques are being posted in AP. How to stop this?
Go to Accounts Payables / Open Setup / Configure Accounts Payable. Scroll down to the Document sequencing section. Change the 'If duplicate numbers' option to Warn or Don't allow. This setting will apply to both invoices and cheques. This will stop any invoices or cheques (or adjustments) with the same number from being posted, but will not stop the duplicates if someone changes the number slightly.