Two transactions have been assigned the same purchase number
MYOB Accounting Plus allows you to assign the same purchase number to multiple purchase transactions. Depending upon how your preferences have been set, a message may appear to alert you when you record the transaction, and you'll be given the option to cancel recording and enter a new number. (If this message wasn't displayed and you'd like it to be in the future, open the Preferences window, choose the Purchases view and mark the "Warn for Duplicate PO Numbers on Recorded Bills" option.)
If you didn't intend to assign a duplicate purchase number to the second purchase and you wish to change it, locate the purchase in the Purchases Register window or the Transaction Journal window and click the zoom arrow. The transaction will appear in the Purchases window as you originally entered it. Depending upon how your preferences have been set, you can either edit the purchase number or reverse the transaction and reenter it with the correct purchase number. (Choosing changeable or unchangeable transactions)