An optimal solution would require that your participant provide a unique ID code, and that the server not serve the rest of the experiment if the unique ID code has already been submitted. Unfortunately this goes beyond the scope of PCIbex, which is a client-based javascript engine.
You could try to use the browser’s cookies, or local storage, but your participants could always clear their cache and/or use other browsers to take the experiment again.
Your best shot is probably to tell your participants that only their first submission will be taken into account. This is what I do: if I find more than one submission for a participant, I only grant them credits for the first one and leave the subsequent ones out of my analyses.
Jeremy