Thought that an inline definition might be useful. (Though anybody can wiki it.)

"In computing, data deduplication is a specialized data compression technique for eliminating coarse-grained redundant data. The technique is used to improve storage utilization and can also be applied to network data transfers to reduce the number of bytes that must be sent across a link. In the deduplication process, unique chunks of data, or byte patterns, are identified and stored during a process of analysis. As the analysis continues, other chunks are compared to the stored copy and whenever a match occurs, the redundant chunk is replaced with a small reference that points to the stored chunk. Given that the same byte pattern may occur dozens, hundreds, or even thousands of times (the match frequency is dependent on the chunk size), the amount of data that must be stored or transferred can be greatly reduced."