The following YAML is to my understanding not valid, due to key1 not being unique:

key1: 
  a: "b"
key1: 
  b: "c"

Please see https://stackoverflow.com/questions/47668308/duplicate-key-in-yaml-configuaration-file for details.
I would have expected yaml-rust to report the duplicate to the user as an error.
Do you share that understanding?
If you can give some hints where to fix this, I would be happy to support.