To configure that your voters can vote more than once in your poll, follow these steps:
1) Open the poll creation form
2) Open the "Poll Settings" Section
3) In the "Expire vote" drop-down, select the time after which the user can vote again (you can select between 1 hour, 6 hours, 24 hours or a week)
4) Click Save
Note that when users vote again the previous vote is not removed, so in this way users can contribute multiple votes to the poll over time.