Check for duplicated yaml keys

[Michael Vogt]

In YAML it is not allowed to have duplicated map keys like:
```
map:
 key:
 key:
```
But unfortunately py-yaml and go-yaml.v2 allow this and do not
warn or error. When moving to go-yaml.v3 we noticed this issue.

It is unclear if any real world snaps are affected by this but
we should make sure we at least to not get more.

Attached is a git formated patch with an initial implementation
for this.

[Alex Murray (alexmurray)]

Thanks, merged in https://git.launchpad.net/review-tools/commit/?id=972d2d5d478e92f4cc09375099d4c4b79b2d3ec5 plus a couple fixup commits in https://git.launchpad.net/review-tools/commit/?id=bc98e6be3cf79c7326a5d0f1b4855ca4154ec46f and https://git.launchpad.net/review-tools/commit/?id=f9126ef0c362de75bc626f71010ed80670a8199f.

[Alex Murray (alexmurray)]

This will be available in review-tools in the edge channel soon.