filemaker - I want it to give me a warning when I enter duplicate data into a single field?

Hi, in my filemaker database there is a text field where I enter a product ID for items at my work, then I fill in info that relates to the item I just entered. It is sort of like an inventory database.

Here is what I would like it to do, I would like to have it so incase I type in a product ID two times (as 2 seperate files) then I want the program to red flag me or give me an error saying I already have that product ID in use already.

Is there an easy way to inpliment that?