Restrict duplicate custom record

Hi ON community,

I have a custom record linked/set up as a child record of a purchase order (let's call it supplier acknowledgement). I would like to restrict multiple acknowledgements to the same PO (ie. 1 custom record to 1 PO  relationship only, cannot be 2 to 1). Is there a way this can be achieved via workflow?

I tried creating a custom field in the PO such that when there is an acknowledgement created against it, it will become ticked (via a set field value workflow). And then second workflow to return user error when creating another acknowledgement when the aforementioned custom field in the PO is ticked. However, the action is on the custom record itself (ie. creating the supplier acknowledgement) whereas the PO is a different record type, hence could not automatically set the custom field value to become ticked whenever a new custom record is created. Any ideas would be much appreciated. Thanks!