2026-07-19 · 5 min read · Rahul Chitturi
- json
- api
- debugging
Duplicate keys in JSON objects are easy to miss because many parsers keep the last value and silently discard the earlier one. A payload may look valid while a feature flag, user role, webhook status, or config setting has already been overwritten.
These bugs often appear after manual edits, templated config generation, YAML-to-JSON conversion, or log copy-paste. The syntax passes, but the meaning depends on which parser or serializer touched the document last.
Inspect the raw object shape
Start with the exact bytes sent over the wire or stored in the config file. Pretty formatting makes repeated keys easier to spot before application code collapses them into a single property.
- Compare raw payload text with the parsed object your service receives
- Check whether generators emit the same key from two template branches
- Look for case variants that become duplicates after normalization
- Confirm YAML merge or anchor expansion does not create repeated JSON keys
Make parser behavior explicit
Different stacks may warn, reject, keep the first value, or keep the last value. If your API contract forbids duplicates, enforce that rule at the boundary instead of relying on whichever parser happens to run first.
When writing incident notes, preserve the duplicate-key pattern with fake values. The structure is the bug, and a sanitized fixture can reproduce it without exposing real customer data.
A Flashman workflow
Use the JSON formatter to inspect raw payload shape, the diff tool to compare generated and expected examples, and the YAML to JSON converter when config expansion is part of the path.
End with a fixture that proves where the duplicate was introduced: hand-written config, code generation, YAML merge behavior, request middleware, or response serialization.