Duplicate property names in JSON

JSON does not forbid a repeated property name, and most parsers silently keep the last one. Here is why that is a problem and how to find them.

Open the JSON formatter

What the error looks like

Different engines word this differently. You may see any of these:

No error is raised by JSON.parse
Some strict validators report: duplicate key

Why it happens

The JSON specification says names within an object should be unique, but it does not require a parser to reject a document that repeats one.
In practice JSON.parse keeps the last occurrence and discards the earlier ones, silently.
That silence is the danger: a configuration file with a repeated key does something different from what it appears to say, and nothing warns you.

Example

Invalid

{
  "timeout": 30,
  "retries": 3,
  "timeout": 60
}

Valid

{
  "timeout": 60,
  "retries": 3
}

How this tool reports it

The formatter reports duplicate property names as problems even though the document parses, and tells you which value actually survives. It is the one case where valid JSON is still worth flagging.

Paste your JSON into the formatter to see the exact line and column, or use the validator if you only need to check it.

Worth knowing

Behaviour differs between languages. Some parsers raise an error, some keep the first occurrence rather than the last. Never rely on it.