8. A system, comprising: a data processing apparatus; and a memory coupled to the data processing apparatus having instructions stored thereon which, when executed by the data processing apparatus cause the data processing apparatus to perform operations comprising: obtaining a trained image relevance model that generates relevance measures of images to a query; and re-training the image relevance model, the re-training comprising: generating an aggregation of near duplicate images among a set of training images; associating image selection data of the aggregated near duplicate images with the aggregation of the near duplicate images, the image selection data corresponding to user interactions with the near duplicate images when the near duplicate images were presented as search results; and generating a re-trained image relevance model based on content feature values of each image in the aggregation of near duplicate images and the image selection data associated with the aggregation of near duplicate images.
8. A system, comprising: a data processing apparatus; and a memory coupled to the data processing apparatus having instructions stored thereon which, when executed by the data processing apparatus cause the data processing apparatus to perform operations comprising: obtaining a trained image relevance model that generates relevance measures of images to a query; and re-training the image relevance model, the re-training comprising: generating an aggregation of near duplicate images among a set of training images; associating image selection data of the aggregated near duplicate images with the aggregation of the near duplicate images, the image selection data corresponding to user interactions with the near duplicate images when the near duplicate images were presented as search results; and generating a re-trained image relevance model based on content feature values of each image in the aggregation of near duplicate images and the image selection data associated with the aggregation of near duplicate images. 12. The system of claim 8 , wherein the instructions cause the data processing apparatus to perform operations further comprising: identifying, for each of a plurality of images, a measure of relevance of the image to the query; comparing the measure of relevance of each image to a relevance threshold; and selecting the set of training images based on the comparison, wherein the images associated with the measure of relevance exceeding the relevance threshold are included in the set of training images.