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Duplicate Access-Control-* headers: why browsers reject ambiguous CORS responses
Multiple values for singleton CORS headers create ambiguity; RFCs expect a single effective policy line.
1 min read
corsdebugginghttp
How browsers evaluate
Some browsers pick the first header; others fail the request—never rely on undefined ordering behavior.
Normalize at one layer and strip duplicates at the edge after integration tests.
Detection
curl -I with verbose mode shows repeated headers; compare against what DevTools displays.
Add temporary logging middleware in lower environments to print outgoing header lists per route.