It's somewhat common for multiple test-cases to use the same PDF, for example tests for both `eq` and `text` with one PDF.
Currently the logic in `downloadManifestFiles` doesn't handle duplicate test PDFs, which leads to wasted time/resources by downloading the same PDF more than once. Naturally the effect of this is especially bad when downloading all linked PDFs.
Total number of test PDFs downloaded:
- 507, with `master`.
- 447, with this patch.