Action check functionality

Hi,

I have never used Action check functionality so if u all could help me would be of great help.

We have a scenario where in the Shipment the user adds a LSP Shipment Refnum. I want to add a action check on it that if the user adds the LSP Shipment more than once it should throw an error.

How should i configure this by using Action Check.

Thanks in advance.

Regards,

Vishal Panchal.