A CAS data storage method and apparatus comprising: receiving input data including a succession of data items with corresponding logical addresses at a source CAS data storage space for storage therein and for replication at a destination CAS data storage space, generating a hash key for each data item at the source storage space, comparing respective hash keys with hash keys stored at a hash key storage table, to determine whether respective further data items are already present at the destination storage device; transferring respective data items to the destination storage space if no match is made to a hash key stored at the hash key storage table, but not transferring respective further data items if a match is made to a hash key stored at the hash key storage table, thereby transferring to the destination storage space only unique data items.