Find and extract all rar files in subfolders into the current folder:

find . -name *.rar -exec unrar x {} \;

Find duplicate files:

find . ! -empty -type f -exec md5sum {} + | sort | uniq -w32 -dD

Find duplicate filenames:

find . -mindepth 1 -printf '%h %f\n' | sort -t ' ' -k 2,2 | uniq -f 1 --all-repeated=separate | tr ' ' '/'