diff --git a/README.md b/README.md
index 519a8dd..ff1ca37 100644
--- a/README.md
+++ b/README.md
@@ -152,7 +152,7 @@ Injects a fake request into an HTTP server.
   - `method` - a string specifying the HTTP request method, defaulting to `'GET'`.
   - `authority` - a string specifying the HTTP HOST header value to be used if no header is provided, and the `url`
     does not include an authority component. Defaults to `'localhost'`.
-  - `headers` - an optional object containing request headers.
+  - `headers` - an optional object containing request headers. A header value may be an array of strings to simulate the same header being sent multiple times; the values are aggregated exactly as a Node.js HTTP server would expose them on `request.headers` (`set-cookie` is kept as an array, `cookie` values are joined with `'; '`, other values with `', '`), while every individual value is preserved in `request.rawHeaders`.
   - `cookies` - an optional object containing key-value pairs that will be encoded and added to `cookie` header. If the header is already set, the data will be appended.
   - `remoteAddress` - an optional string specifying the client remote address. Defaults to `'127.0.0.1'`.
   - `payload` - an optional request payload. Can be a string, Buffer, Stream, or object. If the payload is string, Buffer or Stream is used as is as the request payload. Otherwise, it is serialized with `JSON.stringify` forcing the request to have the `Content-type` equal to `application/json`
diff --git a/lib/request.js b/lib/request.js
index 2ddc0f7..0a6009b 100644
--- a/lib/request.js
+++ b/lib/request.js
@@ -31,6 +31,38 @@ function hostHeaderFromURL (parsedURL) {
     : parsedURL.hostname + (parsedURL.protocol === 'https:' ? ':443' : ':80')
 }
 
+// Header field names for which Node's HTTP parser discards duplicate
+// values, keeping only the first occurrence. Mirrors the aggregation
+// performed by `http.IncomingMessage`, see the `message.headers` docs:
+// https://nodejs.org/api/http.html#messageheaders
+const SINGLE_VALUE_HEADERS = new Set([
+  'age', 'authorization', 'content-length', 'content-type', 'etag',
+  'expires', 'from', 'host', 'if-modified-since', 'if-unmodified-since',
+  'last-modified', 'location', 'max-forwards', 'proxy-authorization',
+  'referer', 'retry-after', 'server', 'user-agent'
+])
+
+/**
+ * Aggregate an array of header values the same way Node's HTTP server
+ * combines multiple occurrences of a header line into `request.headers`.
+ *
+ * @param {String} field the lower-cased header field name
+ * @param {String[]} values the individual header values
+ * @return {(String|String[])}
+ */
+function joinHeaderValues (field, values) {
+  if (field === 'set-cookie') {
+    // `set-cookie` is always exposed as an array of its individual values.
+    return values.slice()
+  }
+  if (SINGLE_VALUE_HEADERS.has(field)) {
+    // Duplicates are discarded, only the first value is kept.
+    return values[0]
+  }
+  // `cookie` duplicates are joined with '; ', everything else with ', '.
+  return values.join(field === 'cookie' ? '; ' : ', ')
+}
+
 /**
  * Mock socket object used to fake access to a socket for a request
  *
@@ -110,13 +142,32 @@ function Request (options) {
 
   const headers = options.headers || {}
 
+  // Records the individual values of headers supplied as an array, so the
+  // raw occurrences can be reflected back in `rawHeaders`.
+  const multiValueHeaders = new Map()
+
   for (const field in headers) {
     const fieldLowerCase = field.toLowerCase()
-    if ((fieldLowerCase === 'user-agent' || fieldLowerCase === 'content-type') && headers[field] === undefined) {
+    const value = headers[field]
+    if ((fieldLowerCase === 'user-agent' || fieldLowerCase === 'content-type') && value === undefined) {
       this.headers[fieldLowerCase] = undefined
       continue
     }
-    const value = headers[field]
+    if (Array.isArray(value)) {
+      // An array represents the same header sent multiple times. Aggregate
+      // the values the way Node's HTTP server exposes them on the request.
+      const rawValues = []
+      for (const item of value) {
+        assert(
+          item !== undefined,
+          'invalid value "undefined" for header ' + field
+        )
+        rawValues.push('' + item)
+      }
+      multiValueHeaders.set(fieldLowerCase, rawValues)
+      this.headers[fieldLowerCase] = joinHeaderValues(fieldLowerCase, rawValues)
+      continue
+    }
     assert(
       value !== undefined,
       'invalid value "undefined" for header ' + field
@@ -139,6 +190,9 @@ function Request (options) {
       cookieValues.unshift(this.headers.cookie)
     }
     this.headers.cookie = cookieValues.join('; ')
+    // The cookie header has been rebuilt, so its raw per-value breakdown is
+    // no longer accurate.
+    multiValueHeaders.delete('cookie')
   }
 
   this.socket = new MockSocket(options.remoteAddress || '127.0.0.1')
@@ -184,7 +238,15 @@ function Request (options) {
   }
 
   for (const header of Object.keys(this.headers)) {
-    this.rawHeaders.push(header, this.headers[header])
+    const rawValues = multiValueHeaders.get(header)
+    if (rawValues !== undefined) {
+      // Expand back to one raw entry per supplied value, as Node does.
+      for (const value of rawValues) {
+        this.rawHeaders.push(header, value)
+      }
+    } else {
+      this.rawHeaders.push(header, this.headers[header])
+    }
   }
 
   // Use _lightMyRequest namespace to avoid collision with Node
diff --git a/test/index.test.js b/test/index.test.js
index b4db714..0dca786 100644
--- a/test/index.test.js
+++ b/test/index.test.js
@@ -1754,7 +1754,7 @@ test('correctly handles no string headers', (t, done) => {
       null: 'null',
       string: 'string',
       object: '[object Object]',
-      array: '1,two,3',
+      array: '1, two, 3',
       date: date.toString(),
       true: 'true',
       false: 'false',
@@ -1775,6 +1775,106 @@ test('errors for invalid undefined header value', (t, done) => {
   }
 })
 
+test('aggregates array header values like a real HTTP server', (t, done) => {
+  t.plan(4)
+  const dispatch = function (req, res) {
+    // General headers are joined with ', ' (comma + space), matching Node.
+    t.assert.strictEqual(req.headers['x-multi'], 'first, second')
+    // Numeric values are coerced to strings before joining.
+    t.assert.strictEqual(req.headers['x-num'], '1, 2')
+    // Each supplied value is preserved as a separate rawHeaders entry.
+    t.assert.deepStrictEqual(
+      req.rawHeaders.slice(0, 8),
+      ['x-multi', 'first', 'x-multi', 'second', 'x-num', '1', 'x-num', '2']
+    )
+    res.writeHead(200)
+    res.end()
+  }
+
+  inject(dispatch, {
+    method: 'GET',
+    url: 'http://example.com:8080/hello',
+    headers: { 'x-multi': ['first', 'second'], 'x-num': [1, 2] }
+  }, (err) => {
+    t.assert.ifError(err)
+    done()
+  })
+})
+
+test('keeps set-cookie request header as an array', (t, done) => {
+  t.plan(3)
+  const dispatch = function (req, res) {
+    t.assert.deepStrictEqual(req.headers['set-cookie'], ['a=1', 'b=2'])
+    t.assert.deepStrictEqual(
+      req.rawHeaders.slice(0, 4),
+      ['set-cookie', 'a=1', 'set-cookie', 'b=2']
+    )
+    res.writeHead(200)
+    res.end()
+  }
+
+  inject(dispatch, {
+    method: 'GET',
+    url: 'http://example.com:8080/hello',
+    headers: { 'set-cookie': ['a=1', 'b=2'] }
+  }, (err) => {
+    t.assert.ifError(err)
+    done()
+  })
+})
+
+test('joins array cookie request header with "; "', (t, done) => {
+  t.plan(2)
+  const dispatch = function (req, res) {
+    t.assert.strictEqual(req.headers.cookie, 'x=1; y=2')
+    res.writeHead(200)
+    res.end()
+  }
+
+  inject(dispatch, {
+    method: 'GET',
+    url: 'http://example.com:8080/hello',
+    headers: { cookie: ['x=1', 'y=2'] }
+  }, (err) => {
+    t.assert.ifError(err)
+    done()
+  })
+})
+
+test('discards duplicates of single-value array headers, keeping the first', (t, done) => {
+  t.plan(3)
+  const dispatch = function (req, res) {
+    // `content-type` is a single-value header: Node keeps only the first.
+    t.assert.strictEqual(req.headers['content-type'], 'text/plain')
+    // rawHeaders still lists every supplied occurrence.
+    t.assert.deepStrictEqual(
+      req.rawHeaders.slice(0, 4),
+      ['content-type', 'text/plain', 'content-type', 'application/json']
+    )
+    res.writeHead(200)
+    res.end()
+  }
+
+  inject(dispatch, {
+    method: 'GET',
+    url: 'http://example.com:8080/hello',
+    headers: { 'content-type': ['text/plain', 'application/json'] }
+  }, (err) => {
+    t.assert.ifError(err)
+    done()
+  })
+})
+
+test('errors for an undefined value inside an array header', (t, done) => {
+  t.plan(1)
+  try {
+    inject(() => {}, { url: '/', headers: { 'header-key': ['a', undefined] } }, () => {})
+  } catch (err) {
+    t.assert.ok(err)
+    done()
+  }
+})
+
 test('example with form-auto-content', (t, done) => {
   t.plan(4)
   const dispatch = function (req, res) {