If you import data enrichment * you can set a column that allows respondents login. If respondents are loaded via data enrichment, you can indicate be whether only one column or a combination of two columns is leading being able to participate in a study. In this way, one respondent can have several times to participate in a survey. This is useful, for example, for continuous surveys.
When importing the data enrichment, for example, select the e-mail address at "Login 1" and at "Login 2" a date or a unique code. As long as the combination between "Login 1 "and" Login 2 "is unique, respondents can access a survey infinitely often Participate.