9. The system of claim 8 , wherein two questions are equivalent if the two questions have an equivalent meaning.
8. A system comprising one or more computers and one or more storage devices storing instructions that when executed by the one or more computers cause the one or more computers to perform operations 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.