sql: json builtin object functions allows duplicate keys

**Describe the problem**

JSON built in functions to create JSON objects succeed to create object even if two similar keys are introduced.

**To Reproduce**

```
select json_build_object('k1', 'v1', 'k1', 'v2')
```

results with 

```
{k1: "v2"}
```

while I would expect to get an error instead since input obviously is not correct.

/cc @jordanlewis 
