Any data resident on an Avamar system is already deduplicated relative to data on that Avamar system.  If that data is replicated to another empty single node Avamar grid then it will take up the same amount of space as on the original node

Bear in mind though that multi-node grids need to create parity stripes for data redundancy in case of node failure so if you replicate data to a multi-node grid, more data will be written.  The relative amount of space taken up by parity data will depend on the number of nodes in the grid and the version of Avamar.  Generally speaking, the later the version of Avamar and the higher the number of nodes in the grid, the less additional space will be needed per node for the parity data.

If the replication target (system B) was not empty then the data being sent from the replication source (system A) may have commonality with the data already resident on system B.