return an error on duplicate keys in configuration

[Geal]

when there are duplicate keys in the configuration files, serde-yaml only takes the last one. We should return an error instead. A solution is outlined here: https://github.com/dtolnay/serde-yaml/issues/150

[BrynCooke]

~~Is there a specific case where this happens? I checked a couple of locations and it looks like the json schema catches duplicates.~~ Found one:

telemetry:
  tracing:
    trace_config:
      attributes:
        foo: bar
        foo: bar