Paste your JSON to find duplicate keys across all nesting levels.
What Is a JSON Duplicate Key?
A duplicate key occurs when the same property name appears more than once within the same JSON object. While the JSON specification (RFC 8259) states that object names should be unique, most parsers won't throw an error when they encounter duplicates. Instead, they silently discard all but the last value — which means data can be lost without warning.
This makes duplicate keys particularly dangerous in configuration files, API responses, and data pipelines. A misplaced copy-paste or a careless merge can introduce a duplicate key that quietly overwrites the value you intended to keep.
How to Use This Tool
Paste raw JSON into the editor above. It can be minified or pretty-printed — the tool handles both. You can also click Sample to load an example with intentional duplicates.
The tool scans every nested object in your JSON and flags any key that appears more than once within the same scope. Results include the key name, its full path, the number of occurrences, and the line numbers where each one appears.
Use the Copy Clean JSON button to grab a deduplicated version of your JSON (the last value for each key is kept, matching standard parser behavior). Or use the line numbers to jump to the source and resolve conflicts manually.
Why Duplicate Keys Are a Problem
Because most JSON parsers accept duplicate keys without complaint, bugs caused by them tend to be silent and hard to trace. Here are common scenarios where they cause real issues:
- Config files — A duplicated setting in a
package.json,tsconfig.json, or app config can override an earlier value, causing builds to fail or features to behave unexpectedly. - API responses — Serializers or hand-built JSON responses with duplicate keys can deliver inconsistent data depending on which parser the client uses.
- Data migration — Merging JSON documents from different sources can introduce overlapping keys that silently drop values during import.
- Security — Duplicate keys have been used in real-world exploits where a parser's "first wins" or "last wins" behavior is leveraged to bypass validation checks.
Features
- Deep nesting support — Scans every level of your JSON, including objects inside arrays.
- Line-level reporting — See exactly which lines contain each duplicate so you can find and fix them fast.
- Full path display — Each result shows the complete dot-notation path to the affected key.
- One-click deduplication — Copy a cleaned version of your JSON with duplicates resolved automatically.
- Format button — Pretty-print minified JSON before scanning for easier reading.
- Client-side only — Your JSON never leaves your browser. Nothing is sent to a server.