A method, system and computer program product for backing up client's data on remote data storage. A client-server infrastructure having multiple clients connected to the backup server is provided. There are two data storages devices connected to the server: data files repository and user data configurations repository. The data files repository contains copies of all data items (i.e., files) that had been stored by each client. The data files repository also contains a hash table with hash values corresponding to data items stored in the data repository. The file attributes corresponding to the data item are stored in the user data configurations repository. Further validation of files having identical hashes is performed on the client using file attributes. Backup of duplicate files is avoided.