Custom message error when duplicating existing key

Hi, I have a project installed with the following features

CRM Suite version 7.2.1
Sugar CRM version 6.5.20

I would like to be able to change the default error and make it more readable for the user when I duplicate a “product label”. That I can put a control in the code so that it looks for if the code of the label exists and if it exists to tell the user “Please enter another code for the label”. My question is given the structure of the module that you see on the attached screen, where you could do this.

Attached screenshots.

Thanks in advance

Since that is on-screen message, I believe you should try a validation by injecting Javascript into the view, something in this line:

Thank you!.