Strangely, you are the second person to ask for this today. The other person making the request was from South Africa. Here is a copy and paste of my previous answer.
Your raise a good point. For questions with variants, (like STACK, but also Moodle's calculated question type, and some of the custom question types) it would be sensible to add one question to the quiz multiple times, and Moodle should be enhanced so that is possible.
This is an issue for how you build the quiz from questions, not for the questions themselves. Therefore, the parts of Moodle that would be affected are just the Edit quiz screen, and what happens when the student clicks 'Start attempt'.
(Well, in the past there was another issue, which was that the back-end database structure would have had a nervous break-down in you added the same question to the quiz twice, but that area was fixed in Moodle 2.1.)
(Oh, and the quiz statistics report might also need to be adjusted to handle the same question appearing in the quiz in different places, but it already handles other similar cases correctly.)
So, when might the Edit quiz page be enhanced to support this? Well, as it happens the OU is planning to do some work there over the next six months. However, this specific feature request is not on our todo list. I will bear this in mind, and see if we can slip it in with the other work. It was first requested a long time ago: MDL-15596.
