Automatically mirror a directory in Ubuntu

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http://askubuntu.com – Running Ubuntu 12.04 I have a library of personal files and documents stored in Dropbox. I want the Dropbox folder to be automatically mirrored to a directory in the local machine, as a backup to the backup. I want it to be done automatically. In other words, each time I add a file/folder to the Dropbpx, that file/folder is copied automatically to the second location on the local drive. Any ideas, tips or advices would be appreciated. Best regards, Ammar (HowTos)