How do I do this?

It should be a workflow rule. If the record exists on the sheet, generate an error message and return.

The example you sent only works if the data is on the device, but if it is not there has no effect.

Effectively the validation must be done when the data is already stored in the sheet. But according to the logic of the app, if the data already exists, the information of the last user who entered it is updated, so the last user is the one who wins....

Thanks. For the moment I have to make the app with Java. I hope this funcinality is included quickly, it really is not worth having to create something from scratch for not having such a basic validation.