Stackexchange is also a collection of questions, some of which have been marked as duplicates by site-adminsitrators. I downloaded a data dump that contains the questions sorted by subject. I picked the chemistry subject because MEsH includes many chemistry-related concepts. There are 660 questions that have been marked as duplicates (class 1). I generated a further 861 pairs of non-duplicates (class 2) by looking for pairs of questions that aren't marked as either duplicates or related.