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d9e9a667a8
Because both strings and symbols keys are serialized the same,
it always has been possible to generate documents with duplicated
keys:
```ruby
>> puts JSON.generate({ foo: 1, "foo" => 2 })
{"foo":1,"foo":2}
```
This is pretty much always a mistake and can cause various
issues because it's not guaranteed how various JSON parsers
will handle this.
Until now I didn't think it was possible to catch such case without
tanking performance, hence why I only made the parser more strict.
But I finally found a way to check for duplicated keys cheaply enough.