Answer once means that each participant only has to answer the survey once i.e. the application keeps track of who had answered and who hasn't. 

Thank You very much. If this check box is untick can any user answer Survey more than one time?

Yes. And if you tick 'Always start as new' it means that they can answer it multiple times without amending the original answers. 

Thank You very much for your quick reply.

But there is a scenario where user cannot answer Survey more than one time even though he unticked  Answer Once check box.

When he try to answer survey second time pop up massage box saying Please contact System Administrator. Most interesting part of this scenario is System Administrator faced this problem.

Can you help me on this.

Is this when you are running ‘Test Survey’ or is it a published survey? Is this for Apps 8?

This is a survey created TEST Data base. I have created it and published it but still have the same problem.

Yes this is App8.