Only use the last value when parsing duplicate cli arguments (#19416)

## Summary

When parsing cli args, 'mri' returns an array for duplicate args, which
causes unexpected results. All current arguments assume single values
only. Tailwind v3 works the same way.

## Test plan

Updated tests.

---------

Co-authored-by: Jordan Pittman <jordan@cryptica.me>

CHANGELOG.md

@@ -22,6 +22,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Try to canonicalize any arbitrary utility to a bare value ([#19379](https://github.com/tailwindlabs/tailwindcss/pull/19379))
 - Validate candidates similarly to Oxide ([#19397](https://github.com/tailwindlabs/tailwindcss/pull/19397))
 - Canonicalization: combine `text-*` and `leading-*` classes ([#19396](https://github.com/tailwindlabs/tailwindcss/pull/19396))
+- Correctly handle duplicate CLI arguments ([#19416](https://github.com/tailwindlabs/tailwindcss/pull/19416))
 
 ### Added
 

packages/@tailwindcss-cli/src/utils/args.test.ts

@@ -17,6 +17,38 @@ it('should be possible to parse a single argument', () => {
   `)
 })
 
+it('should only return the last value for duplicate arguments', () => {
+  expect(
+    args(
+      {
+        '--output': { type: 'string', description: 'Output file' },
+      },
+      ['--output', 'output.css', '--output', 'override.css'],
+    ),
+  ).toMatchInlineSnapshot(`
+    {
+      "--output": "override.css",
+      "_": [],
+    }
+  `)
+})
+
+it('uses last value when flag with "-" is supplied multiple times', () => {
+  let result = args(
+    {
+      '--output': { type: 'string', description: 'Output file', alias: '-o' },
+    },
+    ['--output', 'output.css', '--output', '-'],
+  )
+
+  expect(result).toMatchInlineSnapshot(`
+    {
+      "--output": "-",
+      "_": [],
+    }
+  `)
+})
+
 it('should fallback to the default value if no flag is passed', () => {
   expect(
     args(

packages/@tailwindcss-cli/src/utils/args.ts

@@ -77,9 +77,17 @@ export function args<const T extends Arg>(options: T, argv = process.argv.slice(
   let parsed = parse(argv)
 
   for (let key in parsed) {
-    if (parsed[key] === '__IO_DEFAULT_VALUE__') {
-      parsed[key] = '-'
+    let value = parsed[key]
+
+    if (key !== '_' && Array.isArray(value)) {
+      value = value[value.length - 1]
     }
+
+    if (value === '__IO_DEFAULT_VALUE__') {
+      value = '-'
+    }
+
+    parsed[key] = value
   }
 
   let result: { _: string[]; [key: string]: unknown } = {