Disallow duplicates in configs

[neongreen]

• Duplicate domains are not allowed: https://github.com/lyft/ratelimit/blob/master/test/config/config_test.go#L174-L183
• Duplicate keys are not allowed: https://github.com/lyft/ratelimit/blob/master/test/config/config_test.go#L196-L205 (can use https://hackage.haskell.org/package/yaml-0.11.2.0/docs/Data-Yaml.html#v:decodeFileWithWarnings)

[mdimjasevic]

We have handled the case of duplicate domains. I'll check what is the case with duplicate keys, but my hunch is that such a configuration will give a failure during parsing.