Hello, is there is a way we can change the settings to alert us if a purchase order number has been used twice?

Hi liz31, 

The first thing I would like to verify is if you are using Custom transaction numbers? You can check this using the below steps: 

1. From within your QuickBooks Online Company select the Settings ⚙  icon

2. Select Company Settings 

3. Choose Expenses

4. Select Purchase Orders

5. Tick Custom transaction numbers

6. Save 

If you are using Custom transaction numbers and attempt to enter a duplicate transaction number you will see a message saying 'A purchase order with number xxxx already exists. Do you want to save anyway?' Alternatively you can review the transaction numbers by running the Purchase Order List Report from the Reports section. 

Please let me know if this helps or if you have any other questions at all? 

Rebecca