Checking if the CPF is unique with JQuery validate remote

I'm making a form that checks whether the CPF already exists in a database. In my JS I use the JQuery Validate library

$('#form').validate({
        rules:{          
            cpf: {
                required: true,
                cpfBR: true,
                remote: "cpfunico.php"
            },
       },
        messages: { 
            cpf: { 
                remote: "CPF já cadastrado." 
           }
       },     
});

In the ServerSide file I have the following requisition:

 if(procuraCpf($_GET['cpf'])){
       echo true;
       // ja tentei também com echo (json_encode(true));
   } else {
        echo false;
    } 

The search function Cpf () returns if found cpf in the database and returns to JQuery with a true or false message.

The problem is when the server returns a cpf already registered, and gives the message via Validate. After that all the cpf's will give the message "CPF already registered". I inspected the code and identified that JQuery Validate issues the message before it even requests the database to see if the cpf exists. I do not understand why this happens.