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‎lib/child_process.js‎
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‎test/parallel/test-child-process-env.js‎
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‎test/parallel/test-child-process-env.js‎
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@@ -43,6 +43,10 @@ the first one case-insensitively matching `PATH` to perform command lookup.
 This may lead to issues on Windows when passing objects to `env` option that
 have multiple variants of `PATH` variable.
 
+On Windows Node.js will sanitize the `env` by removing case-insensitive
+duplicates. Only first (in lexicographic order) entry will be passed to the
+child process.
+
 The [`child_process.spawn()`][] method spawns the child process asynchronously,
 without blocking the Node.js event loop. The [`child_process.spawnSync()`][]
 function provides equivalent functionality in a synchronous manner that blocks
 
@@ -29,6 +29,7 @@ const {
   ObjectDefineProperty,
   ObjectPrototypeHasOwnProperty,
   Promise,
+  Set,
 } = primordials;
 
 const {
@@ -524,8 +525,27 @@ function normalizeSpawnArguments(file, args, options) {
     env.NODE_V8_COVERAGE = process.env.NODE_V8_COVERAGE;
   }
 
+  let envKeys = [];
   // Prototype values are intentionally included.
   for (const key in env) {
+    envKeys.push(key);
+  }
+
+  if (process.platform === 'win32') {
+    // On Windows env keys are case insensitive. Filter out duplicates,
+    // keeping only the first one (in lexicographic order)
+    const sawKey = new Set();
+    envKeys = envKeys.sort().filter((key) => {
+      const uppercaseKey = key.toUpperCase();
+      if (sawKey.has(uppercaseKey)) {
+        return false;
+      }
+      sawKey.add(uppercaseKey);
+      return true;
+    });
+  }
+
+  for (const key of envKeys) {
     const value = env[key];
     if (value !== undefined) {
       envPairs.push(`${key}=${value}`);
 
@@ -36,7 +36,9 @@ const env = {
   'HELLO': 'WORLD',
   'UNDEFINED': undefined,
   'NULL': null,
-  'EMPTY': ''
+  'EMPTY': '',
+  'duplicate': 'lowercase',
+  'DUPLICATE': 'uppercase',
 };
 Object.setPrototypeOf(env, {
   'FOO': 'BAR'
@@ -65,4 +67,11 @@ child.stdout.on('end', mustCall(() => {
   assert.ok(!response.includes('UNDEFINED=undefined'));
   assert.ok(response.includes('NULL=null'));
   assert.ok(response.includes(`EMPTY=${os.EOL}`));
+  if (isWindows) {
+    assert.ok(response.includes('DUPLICATE=uppercase'));
+    assert.ok(!response.includes('duplicate=lowercase'));
+  } else {
+    assert.ok(response.includes('DUPLICATE=uppercase'));
+    assert.ok(response.includes('duplicate=lowercase'));
+  }
 }));