1. A method of maintaining a signature repository accessible by multiple client computing devices in a data storage system, the method comprising: tracking storage of data units corresponding to primary data generated by one or more applications executing on a plurality of client computing devices that form a primary storage subsystem, the primary data for each of the client computing devices stored in a data store associated with the respective client computing device, the primary storage subsystem in communication with a secondary storage subsystem that is separate from the primary storage subsystem and is configured to maintain secondary copies of at least some of the primary data; generating, by a signature agent executing on one or more processors in the primary storage subsystem, signatures corresponding to the plurality of tracked data units; and maintaining a signature repository including a signature block for at least each unique signature of the generated signatures, where each signature block comprises: the unique signature; and one or data unit entries, each entry corresponding to a copy of the data unit associated with the unique signature that is stored in the primary storage subsystem, wherein each entry indicates which of the plurality of client computing devices stores the corresponding copy of the data unit, wherein at least some of the signature blocks include at least a first entry indicating that a first client computing device of the plurality of client computing devices stores a copy of the data unit and a second entry indicating that a second client computing device of the plurality of client computing devices stores a copy of the data unit.