Raw JSON text — nested objects fully supported
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Paste JSON and click Detect Duplicates// scan json for duplicate keys that break parsers
Scan raw JSON text for duplicate object keys instantly. Detect conflicts that cause silent data loss in parsers. Free, browser-based, no upload required.
No JSON scanned yet
Paste JSON and click Detect DuplicatesCopy raw JSON text from your file, API response, or editor into the input area.
Enable nested scanning and case-insensitive mode if needed.
Instantly see every duplicate key with its path and occurrence count.
JSON does not forbid duplicate keys — but most parsers silently keep only the last value, causing data loss that's nearly impossible to trace. This tool parses the raw JSON text character-by-character to catch every duplicate before your app does.
The JSON specification (RFC 8259) says duplicate keys should be avoided but doesn't forbid them. The problem is that each parser handles them differently — some keep the first value, some keep the last, some merge, some throw an error. This unpredictability causes silent data corruption that can be very hard to debug.
No. All processing is done entirely in your browser using JavaScript. Your JSON never leaves your machine. No data is sent to any server.
When enabled, the tool recursively checks every object at every depth level — not just the top-level keys. So if a nested array contains objects with duplicate keys, those are caught too.
In case-insensitive mode, keys like "Name" and "name" are treated as duplicates. This is useful when your target system normalizes key casing (common in SQL databases and some ORMs).
The tool handles several megabytes of JSON comfortably in modern browsers. For extremely large files (10MB+), performance depends on your device. There is no hard size limit enforced by the tool.
This tool is a detector, not a fixer. Once you see which keys are duplicated and where, you can manually resolve them in your JSON editor. Use the JSON Formatter tool to view your cleaned JSON with proper indentation.
JSON (JavaScript Object Notation) is designed as a key-value data format where each key in an object should be unique. However, neither the original JSON specification (RFC 4627) nor its successor (RFC 8259) strictly prohibits duplicate keys — they merely state that keys "SHOULD be unique." This creates a dangerous gray zone that routinely bites developers.
When a JSON object contains the same key more than once, what happens? It depends entirely on the parser:
JSON.parse() — silently keeps the last value, discarding all earlier ones.json.loads() — also keeps the last value by default.JsonException.encoding/json — keeps the last value and doesn't warn.The last-wins behavior is the most dangerous because your code runs without error while silently losing data. You pass a JSON config file with a duplicate "timeout" key — one set to 30 seconds, one set to 5 seconds. Your production application uses 5 seconds and you never know why requests are timing out.
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Standard JSON parsers cannot detect duplicate keys because they are designed to produce a parsed object, and by the time the object is built, duplicate keys are already merged. To find duplicates, you need to inspect the raw JSON text before parsing.
This tool implements a custom JSON tokenizer that walks through your raw JSON character-by-character, tracking each object's keys in a separate map. When a key is encountered that already exists in the current object's map, it is flagged as a duplicate along with its full path (e.g., root → users[2] → address → city).
The nested scan feature extends this to every object at every depth in the document, including objects inside arrays. This is critical because a top-level scan would miss the majority of real-world duplicates, which typically occur inside deeply nested structures returned by APIs or databases.
Duplicate keys rarely appear in hand-written JSON. They almost always originate from code. The most common sources include:
Object.assign() can produce duplicates if both have the same key.Once the tool identifies your duplicates and their paths, you have several options depending on the source:
Not all systems treat duplicate keys the same way. If you are sending JSON to a browser-based client, JavaScript's JSON.parse() will silently take the last value. If you are sending to a Python backend, it will also take the last value. If your data is going to a strict XML-based system or some Java configurations, it may throw an error. This inconsistency is exactly why you should eliminate duplicates at the source rather than relying on downstream behavior.
There is one legitimate use case for technically-duplicate keys: HTTP headers (which share a format similar to JSON in some contexts). In that context, duplicate header names are allowed and have defined semantics. But in standard JSON data objects — configuration, API responses, database records — there is no good reason to have duplicate keys, and they should always be treated as bugs.
eslint-plugin-json that can flag duplicate keys in checked-in JSON files.This JSON Duplicate Key Detector is a quick, zero-install way to check any JSON text before it causes problems in production. Paste, scan, fix — it takes seconds and can save hours of debugging.