There is currently no way to compress an file.tar into file.tar.gz by only adding the .gz part, ouch will detect two extensions, tar and gz, repeating the .tar process for that file that was already in that format.

So basically, ouch compress file.tar.gz file.tar.gz.xz generates a file of format .tar.gz.tar.gz.xz, instead of .tar.gz.xz.