As long as a user is working on the survey job, he can see and open your link. This is necessary for the case of technical issues that might force a user to start again. A user can also leave the job for reasons of lacking time or also technical issues for instance, and come back later to start a new job of your survey. 

If you want to prevent users opening your link or taking part several times, please set your tool up to remember and/or block users, for example via an IP address check !