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### What does this PR do? Fixes https://github.com/oven-sh/bun/issues/21201. Ran into this today during a migration from node->bun. TLDR if you set `Transfer-Encoding: chunked` via `res.writeHead()`, Bun sends the header twice because two layers independently add it: 1. `writeFetchHeadersToUWSResponse` (NodeHTTP.cpp) writes the user's headers to the response buffer 2. uWS's `HttpResponse::write()` auto-inserts `Transfer-Encoding: chunked` since no flag indicates it was already set Added a `HTTP_WROTE_TRANSFER_ENCODING_HEADER` flag copying the pattern for `Content-Length` and `Date` deduping, checked before auto-insertion in `write()`, `flushHeaders()`, and `sendTerminatingChunk()`. ### How did you verify your code works? Minimal repro: ``` import http from "node:http"; import net from "node:net"; http.createServer((_, res) => { res.writeHead(200, { "Transfer-Encoding": "chunked" }); res.write("ok"); }).listen(0, "127.0.0.1", function () { const s = net.createConnection(this.address().port, "127.0.0.1", () => s.write("GET / HTTP/1.1\r\nHost: x\r\n\r\n")); s.on("data", (d) => { console.log(d.toString().split("\r\n\r\n")[0]); s.destroy(); this.close(); }); }); ``` Node working (showing header once) ``` $ node --version v22.21.1 $ node /Users/mamps/code/labs/conway/tmp/bun-duplicate-te.mjs HTTP/1.1 200 OK Transfer-Encoding: chunked Date: Tue, 24 Feb 2026 06:14:50 GMT Connection: keep-alive Keep-Alive: timeout=5 ``` Bun bug (duplicate header): ``` $ bun --version 1.3.9 $ bun bun-duplicate-te.mjs HTTP/1.1 200 OK Transfer-Encoding: chunked Date: Tue, 24 Feb 2026 06:13:55 GMT Transfer-Encoding: chunked ``` Bun fixed: ``` $ ./build/debug/bun-debug --version 1.3.10-debug $ BUN_DEBUG_QUIET_LOGS=1 ./build/debug/bun-debug /tmp/bun-duplicate-te.mjs HTTP/1.1 200 OK Transfer-Encoding: chunked Date: Tue, 24 Feb 2026 06:15:53 GMT ```