fix: Prevent duplicate file entries in modulesCode (#82550)

Fixes: #82515

When global-error, and presumably other similar files are defined by
users,`next build` (Webpack) outputs this to
`.next/server/app/page.js`(removed some values for brevity, see OP for
more):

```js
    layout: [/* */],
    "global-error": [() => Promise.resolve().then(c.bind(c, 6076)), "/my-app/app/global-error.tsx"],
    "global-error": [() => Promise.resolve().then(c.bind(c, 6076)), "/my-app/app/global-error.tsx"],
    "not-found": [/* */],
    forbidden: [/* */],
    unauthorized: [/* */],
    metadata: { /* content */ }
```

Since this is then loaded into memory (copied to a JS file that's later
parsed - rather), the last `global-error` key is used ( `{a: 0, a: 100}`
is just `{a: 100}`) - In this case though, the duplicate keys have the
same value too which makes this a **nice** to fix.

This PR does a `reduceRight` pass checking if we have seen this `file`
already, if so, skip. That should be safe enough to not introduce
regressions.

Verified Turbopack builds just for the sake of it - no "issue" there.

Author: icyJoseph

packages/next/src/build/webpack/loaders/next-app-loader/index.ts

@@ -287,7 +287,7 @@ async function createTreeCodeFromPath(
 
       // Fill in the loader tree for all of the special files types (layout, default, etc) at this level
       // `page` is not included here as it's added above.
-      const filePaths = await Promise.all(
+      const filePathEntries = await Promise.all(
         Object.values(FILE_TYPES).map(async (file) => {
           return [
             file,
@@ -302,6 +302,9 @@ async function createTreeCodeFromPath(
           ] as const
         })
       )
+      const filePaths = new Map<ValueOf<typeof FILE_TYPES>, string | undefined>(
+        filePathEntries
+      )
 
       // Only resolve global-* convention files at the root layer
       if (isRootLayer) {
@@ -313,7 +316,7 @@ async function createTreeCodeFromPath(
         }
         // Add global-error to root layer's filePaths, so that it's always available,
         // by default it's the built-in global-error.js
-        filePaths.push([GLOBAL_ERROR_FILE_TYPE, globalError])
+        filePaths.set(GLOBAL_ERROR_FILE_TYPE, globalError)
 
         // TODO(global-not-found): remove this flag assertion condition
         //  once global-not-found is stable
@@ -326,11 +329,11 @@ async function createTreeCodeFromPath(
           }
           // Add global-not-found to root layer's filePaths, so that it's always available,
           // by default it's the built-in global-not-found.js
-          filePaths.push([GLOBAL_NOT_FOUND_FILE_TYPE, globalNotFound])
+          filePaths.set(GLOBAL_NOT_FOUND_FILE_TYPE, globalNotFound)
         }
       }
 
-      let definedFilePaths = filePaths.filter(
+      let definedFilePaths = Array.from(filePaths.entries()).filter(
         ([, filePath]) => filePath !== undefined
       ) as [ValueOf<typeof FILE_TYPES>, string][]
 
@@ -403,7 +406,7 @@ async function createTreeCodeFromPath(
             : parallelSegmentKey
 
       const normalizedParallelKey = normalizeParallelKey(parallelKey)
-      let subtreeCode
+      let subtreeCode: string | undefined
       // If it's root not found page, set not-found boundary as children page
       if (isNotFoundRoute) {
         if (normalizedParallelKey === 'children') {