Duplicate keys in YAML are overwritten with the last key

[bernhardschaefer]

yacs uses safe_load() from pyyaml.

There is a downstream bug with duplicate keys, in the current implementation pyyaml only considers the last key occurrence and does not produce any warning/error: yaml/pyyaml#165
The linked issue contains discussions about workarounds.

This leads to very subtle errors, see e.g.
facebookresearch/maskrcnn-benchmark#545

[rbgirshick]

Thanks for flagging this issue. I agree that yacs should check for duplicate keys and raise an exception if any are found.