Grid: Advise error message when duplicate question is suppressed

This may seem silly, but bare with me. I had a paper grid questionnaire that I was transferring to this module, with a series of questions like this:

Topic A relevance: 1 2 3 4 5
- Helpfulness: 1 2 3 4 5
Topic B relevance: 1 2 3 4 5
- Helpfulness: 1 2 3 4 5
etc.

I was stumped for a while when some of these questions "disappeared" from my form after I entered them. It may seem obvious that you can't have multiple questions called "- Helpfulness", but sometimes when transferring data mindlessly like this, it fails to hit you. I was frustrated for quite a while trying to bug hunt, before the obvious hit me. I would advise that Drupal give an error message if someone tries to enter duplicate questions in a grid as I did.