RequestContext runs twice

I want to send a function of Js from a Jsf bean with RequestContext, but for some strange reason the function is executed twice.

This is the code of my xhtml:

<h:form >
    <p:commandButton class="btn" 
        value="Llamar js Function" 
        action="#{developerController.foo}"/>
</h:form>

This is my bean code:

@Named(value = "developerController")
@SessionScoped
public class DeveloperController implements Serializable {

@EJB
QuoteManager quoteManager;

public DeveloperController() {

}

public void foo() {
    RequestContext requestContext = RequestContext.getCurrentInstance();
    requestContext.execute("alert('Hola');");
}


}

What should I do to have the "alert ()" run one instead of twice?

I'm using primefaces 6.0 and Glassfish 4.1