Duplicate key on output

Assuming two different keys where their json output value is similar an unexpected stringified json can be emitted.

A trivial example is:

```clojure
(jsonista.core/write-value-as-string {:a 1 "a" 2})
;; => "{\"a\":1,\"a\":2}"
```

I can theorize why this is "ok" (mostly performance) but just wanted to ask if it's an issue or acceptable (following a bug I encountered with it in my code).

Thanks.