Bug: duplicate command-line training flags silently parsed

[JRMeyer]

Current behavior: If a flag is duplicated at the CLI and passed to train.py, there is no warning or error message, and the last setting is saved. This was first observed with the --augment flag where the effect results in only the last augmentation being applied. The behavior was replicated with --reverse_train true --reverse_train false, with the final result in flags.txt being --reverse_train false

Expected behavior: An error message stating that a flag has been duplicated, and aborting the training attempt.

[reuben]

This requires modifying Coqpit to handle it, since argparse doesn't have any super simple way to achieve this across the board.