[5.x] Fix existing field validation with prefixed fieldset imports (#13551)

Author: duncanmcclean

src/Http/Controllers/CP/Fields/FieldsController.php

@@ -74,7 +74,11 @@ function ($attribute, $value, $fail) use ($request) {
                         ->when($request->has('id'), fn ($collection) => $collection->reject(fn ($field) => $field['_id'] === $request->id))
                         ->flatMap(function (array $field) {
                             if ($field['type'] === 'import') {
-                                return Fieldset::find($field['fieldset'])->fields()->all()->map->handle()->toArray();
+                                return Fieldset::find($field['fieldset'])
+                                    ->fields()->all()
+                                    ->map(fn ($importedField) => ($field['prefix'] ?? '').$importedField->handle())
+                                    ->values()
+                                    ->toArray();
                             }
 
                             return [$field['handle']];