Actually I've seen 5 or more ballots on some problems. Chess.com is trying to set up a database so you could filter exactly what problems you want to work on. If you never get the "smothered mate" for instance, you could choose to work exclusively on those problems. One problem could have a number of themes associated with it however.
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Does anyone know why there are two thumbs ballots on the chess problems? Why two, are they for different aspects? I couldn't find anything in faqs.