Cannot add an entity with a key that is already in use

I'm trying to save data with linq to class. I'm getting following error at line _dbContext.SubmitChanges();:

Cannot add an entity with a key that is already in use

   at System.Data.Linq.ChangeProcessor.TrackUntrackedObjects(MetaType type, Object item, Dictionary`2 visited)
   at System.Data.Linq.ChangeProcessor.TrackUntrackedObjects(MetaType type, Object item, Dictionary`2 visited)
   at System.Data.Linq.ChangeProcessor.TrackUntrackedObjects()
   at System.Data.Linq.ChangeProcessor.SubmitChanges(ConflictMode failureMode)
   at System.Data.Linq.DataContext.SubmitChanges(ConflictMode failureMode)
   at System.Data.Linq.DataContext.SubmitChanges()
   at WebSolution.....UpdateQuestion(Int64 questionId, String Title, 

Following source code:

Question Questions = (from a in _dbContext.Questions where a.QuestionID == questionId select a).SingleOrDefault();
               -----
                Questions.UpdatedDate = DateTime.UtcNow;
                Questions.QuestionPrivacy = QuestionPrivacy;
                Questions.IsActive = 'Y';
                Questions.IPAddress = "";
                foreach (var keyword in keywordArray)
                {
                    var questionKeyword = Questions.QuestionKeywords.FirstOrDefault(x => x.QuestionId == Questions.QuestionID && x.KeywordShortName.ToLower() == keyword.ToLower());
                    if (questionKeyword == null)
                    {
                        questionKeyword = new tbl_QuestionKeyword();
                        questionKeyword.KeywordShortName = keyword;
                        questionKeyword.QuestionId = Questions.QuestionID;
                        Questions.QuestionKeywords.Add(questionKeyword);
                    }
                }
                _dbContext.SubmitChanges();