Numeric Entry Drop Down

Thanks in advance - this forum has been so helpful!

We have created the attached work style assessment tool using custom built drop down boxes for numeric entry.  There is an error lightbox slide if the user tries to use the same number twice for the question.  If the user leaves an entry blank, the course doesn't know what to do and locks up.  We've tried to set conditions to activate the same error slide if the users leaves a blank entry but since the numeric entry is blank (my understanding is that an empty numeric default entry is really NaN (not a number) instead of a default numeric value.  Looking for ideas.