nextjs: fix handler overwrite

internal/nextjs/nextjs.go

@@ -144,20 +144,21 @@ func traverse(fs fs.ReadDirFS, fp string, segments []string, parent *node) error
 			name = "[]"
 		}
 
-		var handler http.Handler
-		if !child.IsDir() {
-			handler = httpFileHandler(fs, childPath)
-		}
-
 		// files may share the same name as a directory, so we need to check if the node already exists
 		childNode, ok := parent.sub[name]
 		if !ok {
 			childNode = node{
-				h:   handler,
 				sub: make(map[string]node),
 			}
 		}
 
+		if !child.IsDir() {
+			if childNode.h != nil {
+				return fmt.Errorf("failed to add route %q: route already exists", childPath)
+			}
+			childNode.h = httpFileHandler(fs, childPath)
+		}
+
 		if child.IsDir() {
 			if err := traverse(fs, childPath, append(segments, name), &childNode); err != nil {
 				return err