Apps similar to dupd include Rdfind, which is free and open source. Other options are fdupes, jdupes, duplikaatti and Samanlainen. In total there are six Duplicate File Finders like dupd for Linux, Windows and Mac.
Rdfind is a program that finds duplicate files. It is useful for compressing backup directories or just finding duplicate files. It compares files based on their content, NOT on their file names.
FDUPES is a program for identifying or deleting duplicate files residing within specified directories.
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Remove duplicate files and do it fast. duplikaatti is designed to go through 50 TiB+ of data and hundreds of thousands of files and find duplicate files in few minutes.
Samanlainen is a command line duplicate file remover which uses SHA-512 hashes to identify identical files. It's inspired by Rdfind
