1. A method performed by one or more computers, the method comprising: generating a question graph that includes a respective node for each of a plurality of questions; connecting, with links in the question graph, nodes for questions that are equivalent, comprising: identifying selected resources for each of the plurality of questions based on user selections of search results in response to previous submissions of the question as a search query to a search engine; identifying pairs of questions from the plurality of questions, wherein the questions in each identified pair of questions have at least a first threshold number of common identified selected resources; and for each identified pair, connecting the nodes for the questions in the identified pair with a link in the question graph; receiving a new search query from a user device; obtaining an initial ranking of questions that are related to the new search query; generating a modified ranking of questions that are related to the new search query, comprising, for each question in the initial ranking: determining whether the question is equivalent to any higher-ranked questions in the initial ranking by determining whether a node for the question is connected by a link to any of the nodes for any of the higher-ranked questions in the question graph; and when the question is equivalent to any of the higher-ranked questions, removing the question from the modified ranking; selecting one or more questions from the modified ranking; and transmitting data identifying the selected questions to the user device as part of a response to the new search query.