When using -x to add additional options, i.e., to force a specific engine,
often runs of PRISM are effectively duplicated, e.g., if there are .args files
that select multiple engines.
Using the --skip-duplicate-runs, prism-auto tries to clean up the argument list,
remove switches that don't have an effect and to detect duplicate runs, only actually
executing the first one.
Joachim Klein
f1378fab52