title: Validate whether unique slug already exists before submitting CPT
body: <p>I have an ACF Extended frontend form in place for a CPT. One of the CPT fields populates the slug field on submit.</p>
  <p>At the moment, the form will submit the form regardless of whether the slug is unique. It will automatically add the numerical append eg: /example (if already used), will become /example-1, then /example-2 etc.</p>
  <p>I would like to ensure that the slug is unique before the CPT form can be submitted.</p>
  <p>And if the slug already exists, it could show an error message like 'already taken, please choose another'.</p>
  <p>Could anyone please help me achieve this?</p>