Deduplication

Wikipedia

 * Data deduplication
 * Write Anywhere File Layout
 * ZFS

Linux/file systems

 * Data Deduplication with a Linux Based File System
 * How to Save Disk Space using Btrfs Deduplication
 * The 5 Best Tools to Find and Remove Duplicate Files in Linux
 * How to Delete Duplicate Files in Linux with Fdupes

Within files

 * How to remove duplicate lines inside a text file?