You can select multiple completes, as link type while creating email distribution. But this will always create new response for the same respondent, every time they complete the survey.
But if you want them to override their previous response every time they take new response then, select "Individual link type" in email distribution. In the survey create a email task with retake link, which will send them the retake link every time they complete the survey.

Hi amandaysy ,
If you're sending out the links by email through Qualtrics, it will allow a unique link to be used multiple times if you select the Link Type "Multiple Completes" in the Advanced Options of the email composer. Note that this will create a new response each time the respondent completes the survey; it will not overwrite the previous response. If you want to overwrite the previous response, this has to be done manually for each respondent by the Qualtrics user, using the Retake Response feature in Data & Analysis.