Despite what commenters have said in issue #2350, this phenomenon of Snakemake running the python code twice is a feature, not a bug, and it's not going to be "fixed". Even if the behaviour was changed for the simple -j1 case it will still manifest as soon as you allow parallel jobs or submit your jobs to a HPC cluster (this includes your fixed code, @antje-janosch!)

The actual bug in this code is on line 16. If you replace {tarfile} with "{tarfile}" (only on line 16, not line 12) in the original example you will find that it works, just by adding the two quotes.

The difference is this: {tarfile} means "a python set containing the single fixed string from the variable tarfile", whereas "{tarfile}" is a template string that will act as a placeholder for any filename. These are not the same!