It's  a bug. I logged it:

http://support.dotnetnuke.com/issue/ViewIssue.aspx?id=5632

I will fix it in the next week. For now you can log in as host, go to SQl and run this script:

if exists (select * from dbo.sysobjects where id = object_id(N'{databaseOwner}[{objectQualifier}AddSurveyResult]') and OBJECTPROPERTY(id, N'IsProcedure') = 1)

drop procedure {databaseOwner}{objectQualifier}AddSurveyResult

GO

create procedure {databaseOwner}{objectQualifier}AddSurveyResult

When the new survey comes (version 04.00.80 or higher) out you will just install it and then run the script at the end of this post and you will be set. Right now if you set the survey to be one vote per user, a user can vote twice for the same choice.

When the new survey comes out (it may take 3 weeks to go through the release process) it will prevent this and also fix the bug you found.

if exists (select * from dbo.sysobjects where id = object_id(N'{databaseOwner}[{objectQualifier}AddSurveyResult]') and OBJECTPROPERTY(id, N'IsProcedure') = 1)
 drop procedure {databaseOwner}{objectQualifier}AddSurveyResult
GO

/** Create Stored Procedure **/
create procedure {databaseOwner}{objectQualifier}AddSurveyResult @SurveyOptionID int, @UserID int as
DECLARE @i int
Set @i = (SELECT COUNT(*) AS records
FROM {objectQualifier}SurveyResults
 WHERE (UserID = @UserID) AND (SurveyOptionID = @SurveyOptionID))

 if (@i = 0)
begin
update {objectQualifier}SurveyOptions set Votes = Votes + 1
 where SurveyOptionID = @SurveyOptionID
 insert into {objectQualifier}SurveyResults (SurveyOptionID, UserID)
 values (@SurveyOptionID, @UserID)
 end