Fix instrumentation.ts loading inconsistency between dev and production

- Add validation to detect conflicting instrumentation files (both root and src/)
- Throw error when both root and src instrumentation files exist
- Apply fix to both build process and dev server
- Consistent with middleware/proxy conflict handling

Author: cursoragent

packages/next/errors.json

@@ -1050,5 +1050,7 @@
   "1049": "Turbopack is not supported on this platform (%s/%s) because native bindings are not available. Only WebAssembly (WASM) bindings were loaded, and Turbopack requires native bindings.\\n\\nTo use Next.js on this platform, use Webpack instead:\\n  next dev --webpack\\n\\nFor more information, see: https://nextjs.org/docs/app/api-reference/turbopack#supported-platforms",
   "1050": "Turbopack is not supported on this platform (%s/%s) because native bindings are not available. Only WebAssembly (WASM) bindings were loaded, and Turbopack requires native bindings.\\n\\nTo build on this platform, use Webpack instead:\\n  next build --webpack\\n\\nFor more information, see: https://nextjs.org/docs/app/api-reference/turbopack#supported-platforms",
   "1051": "Turbopack trace server is not supported on this platform (%s/%s) because native bindings are not available. Only WebAssembly (WASM) bindings were loaded, and Turbopack requires native bindings.",
-  "1052": "Turbopack is not supported on this platform (%s/%s) because native bindings are not available. Only WebAssembly (WASM) bindings were loaded, and Turbopack requires native bindings. Use the --webpack flag instead."
+  "1052": "Turbopack is not supported on this platform (%s/%s) because native bindings are not available. Only WebAssembly (WASM) bindings were loaded, and Turbopack requires native bindings. Use the --webpack flag instead.",
+  "1053": "Conflicting instrumentation files detected: \"./%s\" and \"./%s\". Please remove one of them.",
+  "1054": "Conflicting instrumentation files detected: \"%s\" and \"%s\". Please remove one of them."
 }

packages/next/src/build/index.ts

@@ -1215,6 +1215,8 @@ export default async function build(
         .map((file) => path.join(rootDir, file).replace(dir, ''))
 
       let instrumentationHookFilePath: string | undefined
+      let rootInstrumentationHookFilePath: string | undefined
+      let srcInstrumentationHookFilePath: string | undefined
       let proxyFilePath: string | undefined
       let middlewareFilePath: string | undefined
 
@@ -1235,10 +1237,31 @@ export default async function build(
           isAtConventionLevel &&
           fileBaseName === INSTRUMENTATION_HOOK_FILENAME
         ) {
+          if (normalizedFileDir === '/') {
+            rootInstrumentationHookFilePath = rootPath
+          } else if (normalizedFileDir === '/src') {
+            srcInstrumentationHookFilePath = rootPath
+          }
           instrumentationHookFilePath = rootPath
         }
       }
 
+      if (rootInstrumentationHookFilePath && srcInstrumentationHookFilePath) {
+        const cwd = process.cwd()
+        const absoluteRootPath = path.join(
+          rootDir,
+          rootInstrumentationHookFilePath
+        )
+        const absoluteSrcPath = path.join(
+          rootDir,
+          srcInstrumentationHookFilePath
+        )
+
+        throw new Error(
+          `Conflicting instrumentation files detected: "./${path.relative(cwd, absoluteRootPath)}" and "./${path.relative(cwd, absoluteSrcPath)}". Please remove one of them.`
+        )
+      }
+
       if (middlewareFilePath) {
         if (proxyFilePath) {
           const cwd = process.cwd()

packages/next/src/server/lib/router-utils/setup-dev-bundler.ts

@@ -591,6 +591,15 @@ async function startWatcher(
           continue
         }
         if (isInstrumentationHookFile(rootFile)) {
+          if (
+            serverFields.actualInstrumentationHookFile &&
+            serverFields.actualInstrumentationHookFile !== rootFile
+          ) {
+            const existingFile = serverFields.actualInstrumentationHookFile
+            throw new Error(
+              `Conflicting instrumentation files detected: "${existingFile}" and "${rootFile}". Please remove one of them.`
+            )
+          }
           serverFields.actualInstrumentationHookFile = rootFile
           await propagateServerField(
             opts,