1. A computer-implemented method of detecting duplicate documents in a network crawling system, comprising, at a server having one or more processors and memory: constructing a plurality of tables, each table corresponding to a portion of a document address space, storing information identifying documents having a same document content identifier and each identified document having an associated document rank; wherein documents having the same document content identifier have the same content and documents having different document content identifiers have different content; receiving a newly crawled document, such document characterized by a document content identifier and a document rank; reading information stored in the plurality of tables to identify a set of documents sharing the document content identifier of the newly crawled document, and ascertaining an original representative document for the identified set of documents; updating the information stored in at least one of the tables in accordance with the document ranks of the identified set of documents and the newly crawled document; determining a representative document for the newly crawled document and the identified set of documents; indexing the representative document when the representative document is the newly crawled document; and repeating the receiving, reading, updating, determining and indexing operations with respect to a plurality of newly crawled documents, each of which shares a respective document content identifier with a respective set of documents, such that at least some of the newly crawled documents are determined to be representative documents and are indexed.