Hi Ana,
The first column in the results file is already (almost) unique to each participant. You only get duplicates when the same browser on the same device is used multiple times to take the experiment
You can also generate a unique completion code, as described here. Some recruiting platforms also allow you to pass a unique ID as part of the experiment’s URL: here is a guide on how to interface your PCIbex experiment with the recruiting platform Prolific
Let me know if you have questions
Jeremy