Over the years I've used many, many tools intended to solve this problem. In the end, after much frustration, I just use existing tools, glued together in a un*x manner.

  find * -type f -exec md5sum '{}' ';' \
  | tee /tmp/index_file.txt            \
  | gawk '{print $1}'                  \
  | sort | uniq -c                     \
  | gawk '/^ *1 /{ print $2 }          \
  > /tmp/duplicates.txt

  for m in $( cat /tmp/duplicates.txt )
  do
    grep $m /tmp/index_file.txt
    echo ========
    done \
  | less

Tweak as necessary. I do have a comparison executable that only compares sizes and sub-portions to save time, but I generally find it's not worth it.

It takes less time to type this that than it does to remember what some random other tool is called, or how to use it. I also have saved a variant that identifies similar files, and another that identifies directory structures with lots of shared files, but those are (understandably) more complex (and fragile).