If I run depcheck 5 times in a row, it downloads the same packages over and over again. That slows down the development a lot.

Try to adjust koji_directive and koji_utils to:
1. Use some common directory for downloading packages, e.g. /var/tmp/taskotron/koji/.
2. Check if the download is necessary before starting it (that might be already handled by download() function).
3. The download() function used to do some file length checking in AutoQA to better verify that the file is complete. Some RPM checksuming is also an option. Investigate why we removed this functionality and whether we can add it back again.

This will very much speed up depcheck development. Test your patch with depcheck.