linux:

find . -type f -print0 | xargs -0 -n1 md5sum | sort -k 1,32 | uniq -w 32 -d --all-repeated=separate | sed -e 's/^[0-9a-f]*\ *//;'

mac osx:

find . -type f -print0 | xargs -0 cksum | sort | awk '{if($1 == prevsum) {printf("----\n %s\n %s\n", prev, $0);} prev=$0; prevsum=$1;}'