Mathematical Conundrum or Not? Why don't multiple choice questions ever have repeated answers? If you were taking a math test and got to a question that had a repeated answer, you'd walk up to the teacher's desk and show him, and he'd make an announcement like, "Sorry, folks, Question 17 has a typo. Answer E should be '33%'."
Presented with a multiple-choice question, there are several methods you can use to choose an answer. Best method is knowing which answer is correct -- a word which here means "will be graded by the test preparer as correct". If you don't know, or don't think you know, you can go with your gut or choose randomly, and you can also eliminate answers you know are wrong before doing either of those to improve your (subjective) chances. There are some other methods, but the main point is that random choice is a fall-back when no better option is available.
Multiple choice tests are designed to test knowledge or reasoning, not luck. Tests are often designed with answers that will appear tempting if your reasoning is faulty, but not to foil a test taker who is lucky. That test takers have the option of making random choices is interesting, but test design needn't take this into account, and it's not perfectly clear that it can.
You could in fact help the random chooser by repeating correct answers, and people do this sort of thing for comic effect. What are the three most important principles of retail? Location, location, location. There are only two rules for working here: 1 is "Do what I tell you" and 2 is "Do what I tell you".
So here's the answer to the question at the top of this post: multiple choice questions never (deliberately) have repeated answers because a duplicated incorrect answer will not foil the random chooser, while a duplicated correct answer will help him. And the test preparer has no reason to help the random chooser.
The question that remains is whether a duplicated incorrect answer also helps the random chooser by changing his chance of getting the right answer from, say, 1 in 4 to 1 in 3. That would seem to depend entirely on the test taker -- that is, on whether he reads the question at all or just bubbles in something on the answer sheet.
And that opens up the possibility of a gap -- noted by several people in this thread -- between the chance of my picking the answer that is correct, on the one hand, and the chance of the answer I pick being correct. In the usual case, with no duplicated answers, these are identical, by design. But if there are duplicated wrong answers, will the random chooser who reads the questions out-perform the random chooser who doesn't?
If I did the simulation right, I get 16.6162 for the test taker who reads the questions, and 12.4914 for the test taker who doesn't.