Error handling: duplicate file names in multiple submitted yaml files

[dongbohu]

This is not the highest priority, but should be addressed in the future:

When a user submits multiple YAML files, the downloaded files (and their extracted content) may be duplicate. Before saving them in submitted_data directory, we should add extra logic to check the saved filenames and report errors if any of them already exists.

[dongbohu]

Fixed by checking md5sum in catalog.txt.