http: preserve raw header duplicates in writeHead after setHeader calls

lib/_http_server.js

@@ -373,9 +373,18 @@ function writeHead(statusCode, reason, obj) {
         throw new ERR_INVALID_ARG_VALUE('headers', obj);
       }
 
+      // Headers in obj should override previous headers but still
+      // allow explicit duplicates. To do so, we first remove any
+      // existing conflicts, then use appendHeader.
+
+      for (let n = 0; n < obj.length; n += 2) {
+        k = obj[n + 0];
+        this.removeHeader(k);
+      }
+
       for (let n = 0; n < obj.length; n += 2) {
         k = obj[n + 0];
-        if (k) this.setHeader(k, obj[n + 1]);
+        if (k) this.appendHeader(k, obj[n + 1]);
       }
     } else if (obj) {
       const keys = ObjectKeys(obj);

test/parallel/test-http-write-head-after-set-header.js

@@ -0,0 +1,47 @@
+'use strict';
+
+const common = require('../common');
+const Countdown = require('../common/countdown');
+const assert = require('assert');
+const { createServer, request } = require('http');
+
+const server = createServer(common.mustCall((req, res) => {
+  if (req.url.includes('setHeader')) {
+    res.setHeader('set-val', 'abc');
+  }
+
+  res.writeHead(200, [
+    'array-val', '1',
+    'array-val', '2',
+  ]);
+
+  res.end();
+}, 2));
+
+const countdown = new Countdown(2, () => server.close());
+
+server.listen(0, common.mustCall(() => {
+  request({
+    port: server.address().port
+  }, common.mustCall((res) => {
+    assert.deepStrictEqual(res.rawHeaders.slice(0, 4), [
+      'array-val', '1',
+      'array-val', '2',
+    ]);
+
+    countdown.dec();
+  })).end();
+
+  request({
+    port: server.address().port,
+    path: '/?setHeader'
+  }, common.mustCall((res) => {
+    assert.deepStrictEqual(res.rawHeaders.slice(0, 6), [
+      'set-val', 'abc',
+      'array-val', '1',
+      'array-val', '2',
+    ]);
+
+    countdown.dec();
+  })).end();
+}));