Duplicate key exception in user answering stats calculation
java.lang.IllegalStateException: Duplicate key [...] (attempted merging values Optional[[ChoiceQuestionContent ...]] and Optional[[ChoiceQuestionContent ...]])
java.lang.IllegalStateException: Duplicate key [...] (attempted merging values de.thm.arsnova.model.AnswerResult@[...] and de.thm.arsnova.model.AnswerResult@[...])