How to warn the user of unique submission check so they know why their submission is not accepted?

mais sim acontecer da pessoal coloca o mesmo CPF preciso personalizar uma mensagem explicando

como faz?

To change that you would need to change the form warnings.

In English, the warning is shown as:

"Sorry! Only one entry is allowed.  Multiple submissions are disabled for this form."

If you change that text to something else, they will be shown this error.

However, this is going to be shown to your users only when they submit the form, not while they are filling it out - since that would not be possible.

You can see how to change these warning messages here: How to Change Form Warnings

Do let us know if you happen to have any further questions and we would be happy to assist with the same.