Our system is set up such that each participant can have only one submission per study on Prolific Academic. That is, each participant will be listed in your dashboard only once.
However, we obviously can't prevent people from visiting the 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 such as Qualtrics (which we don't have control over).
To prevent duplicate entries in Qualtrics enable the "Prevent Ballot Box Stuffing" option.
If a participant takes your study twice (for whichever reason), then we recommend that you simply delete the second submission from your data set (by checking their participant ID or IP address).