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. 10. The system of claim 8 , wherein: the instructions cause the data processing apparatus to perform operations further comprising generating multiple distinct portions of the set of training images; and each of the multiple distinct portions is generated to include fewer than all of the images in the set of training images.