Our system is set up so that each participant can have only one submission per study on Prolific. That is, each participant will be listed in your dashboard only once, and can only be paid the main study payment once.
However, we can't prevent people from visiting your external survey link more than once after they have opened it for the first time. Sometimes, participants revisit a survey link if, for example, they have missed the completion URL. It is important to distinguish between submissions on Prolific (which we guarantee are unique) and submissions to your survey software (which we don't have control over).
To prevent duplicate submissions in your external survey software, look out for a 'prevent multiple submissions' (or similar) option in your external survey software settings.
If a participant takes your study twice then we recommend that you simply delete the second submission from your data set. You'll be able to match the submissions using the participant's Prolific ID.