JSON Duplicate Key Finder
Detect duplicate keys that can cause data loss
Instant Scan
Detect duplicates in milliseconds
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All processing happens in your browser
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JSON Duplicate Key Checker
JSON.parse silently keeps the last value when a key is repeated. The checker walks every object and tells you exactly where the duplicates live.
Instant
Runs entirely in your browser — no upload, no wait.
100% private
Your JSON never leaves the tab. Nothing is logged.
No signup
Free, unlimited, no account required.
The silent JSON bug
JSON technically allows duplicate keys (the spec only says behavior is "implementation-defined"). Every mainstream parser keeps the last occurrence. The result: data loss with no warning. The checker scans your document and reports each duplicate with its full JSON Pointer path.
How duplicates sneak in
Most often via concatenation — merging two API responses, hand-editing a config, or copying a block of keys without noticing one of them already exists. Code generators occasionally emit duplicates when two source schemas map to the same JSON property.
Reading the report
Each row shows the offending key, the JSON Pointer to the parent object, the values that would be lost, and a quick action to keep the first or the last occurrence.
CI integration
For automated checks, ajv with allowUnionTypes: false and a custom keyword can flag duplicates at validation time. For raw text scanning, the snippet below is enough.
// Lossless reviver — collects duplicates instead of dropping them
const seen = [];
JSON.parse(input, function (key, value) {
if (this && Object.prototype.hasOwnProperty.call(this, key) && seen.indexOf(this) === -1) {
seen.push(this);
}
return value;
});Privacy
All scanning happens in the browser. No upload, no telemetry.