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 a data fingerprint has been seen in arriving data chunks to be written in a storage system, and distinguishes multiply-seen entries for data fingerprints present in at least two data chunks from once-seen entries for data fingerprints present in no more than a single data chunk; retaining higher-frequency entries, while also taking into account recency of data accesses for the higher-frequency entries based on the seen-count attribute and the data access age; 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, wherein a policy is applied for distinguishing multiple seen-count categories based on tracking data access ages of entries in the fingerprint directory for different seen-count categories.