Warehouses have the ability to request a unique PO number for carriers. This is done through the Org Settings tab. 

For PO validation, please check this article. For a unique PO, you must use the Reference number on the Org Settings instead of the custom appointment fields. 

The custom appointment fields tab does not support uniqueness. 

Instead, go to the 'My Organization' tab, access the 'Org Settings', and select the Reference Number tab. 

1. Select YES on 'Show Reference Number Field'
2. Add a Display Name
3. You may add a Helper Text - not mandatory
4. Select YES on 'Require Reference Number Field'
5. Select YES on 'Require Unique Reference Numbers'
6. Click 'Save Org Settings' at the end