app/Filament/Resources/ProductionPlanResource.php

@@ -826,6 +826,39 @@ class ProductionPlanResource extends Resource
                     ->label('Import Production Plans')
                     ->color('warning')
                     ->importer(ProductionPlanImporter::class)
+                    ->fileRules([
+                        fn (): \Closure => function ($attribute, $value, $fail) {
+
+                            $csv = \League\Csv\Reader::createFromPath(
+                                $value->getRealPath(),
+                                'r'
+                            );
+
+                            $csv->setHeaderOffset(0);
+
+                            $itemCodes = [];
+
+                            foreach ($csv->getRecords() as $record) {
+                                $itemCode = trim((string) ($record['Item Code'] ?? ''));
+
+                                if ($itemCode !== '') {
+                                    $itemCodes[] = $itemCode;
+                                }
+                            }
+
+                            $duplicates = collect($itemCodes)
+                                ->duplicates()
+                                ->unique()
+                                ->values();
+
+                            if ($duplicates->isNotEmpty()) {
+                                $fail(
+                                    'Duplicate Item Codes found: ' .
+                                    $duplicates->implode(', ')
+                                );
+                            }
+                        },
+                    ])
                     ->visible(function () {
                         return Filament::auth()->user()->can('view import production plan');
                     }),