I'd come across this before, and it was very annoying. Last night, I downloaded a zip archive of freely-distributed MP3 files, and when I double-clicked it to decompress it, all I got was a .cpgz file. This is apparently a zipped CPIO archive file. Double-clicking that file just created the zip file again, and this was an endless loop

Searching on Google, I saw that plenty of people had come across this problem, and offered a number of suggestions, none of which worked for me. Some articles suggested that the download might have been corrupted, but as this was a very large file, I didn't want to try and download it again.

The solution for me was to use the free The Unarchiver, which has turned out to be a Swiss army knife for decompressing many obscure types of archives. Opening the file with The Unarchiver decompressed it correctly.

I didn't think of it at the time - and I have since deleted the archive - but the cpio command would probably have worked via Terminal as well. In any case, if you encounter this odd archive decompression loop, here's an easy way to solve the problem