lsdup
List files with duplicate contents
lsdup, short for "list duplicates", is a command line tool which can assist in finding and removing files with identical contents.
Unlike many file deduplication tools, this has a "comparison mode" which lets you tell lsdup to "only read the files in this directory, do not consider any for duplicates... but, the files in that other directory may have duplicates, so delete (or move) any of those."
lsdup also has normal deduplication, where, if given a single directory, it will sort the originals from the duplicates.
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