1. A method for detecting data duplication, comprising: maintaining a fingerprint directory comprising one or more entries, each entry including a data fingerprint and a data location for a data chunk; associating each said entry with a seen-count attribute which is an indication of how often the fingerprint has been seen in arriving data chunks; retaining higher-frequency entries, while also taking into account recency of data accesses; and detecting that the data fingerprint for a new chunk is the same as the data fingerprint contained in an entry in the fingerprint directory.