`UnmarshalStrict()` doesn't error on duplicate keys

[aeijdenberg]

I spent yesterday afternoon tracking down a bug in our Concourse configuration whereby the root cause was a YAML file containing:

resource_types:
- name: foo
- name: bar

... hundreds of unrelated lines later ...

resource_types:
- name: baz

where the final resource_types effectively deleted the first.

I thought it would be within the spirit of what UnmarshalStrict() sounds like it should do, if it would return an error if a key is attempted to be set in a map (or struct) that already has value.

PR forthcoming for your consideration...

[aeijdenberg]

Related: #154

[xunwu]

I am facing the same issue here, I think it should throw an error if duplicate keys are found, rather than silently replacing the value

[rogpeppe]

Fixed by #307.