fix: harden postcodes:import against duplicate headers and test collisions

app/Console/Commands/ImportPostcodes.php

@@ -6,7 +6,6 @@ use Illuminate\Console\Attributes\Description;
 use Illuminate\Console\Attributes\Signature;
 use Illuminate\Console\Command;
 use Illuminate\Support\Facades\DB;
-use RuntimeException;
 
 #[Signature('postcodes:import {--file= : Path to ONSPD CSV file}')]
 #[Description('Import UK postcodes (ONSPD) into the local postcodes and outcodes tables')]
@@ -27,7 +26,9 @@ final class ImportPostcodes extends Command
         $handle = fopen($file, 'r');
 
         if ($handle === false) {
-            throw new RuntimeException("Unable to open {$file}");
+            $this->error("Unable to open {$file}.");
+
+            return self::FAILURE;
         }
 
         $header = fgetcsv($handle);
@@ -39,6 +40,17 @@ final class ImportPostcodes extends Command
             return self::FAILURE;
         }
 
+        $headerCounts = array_count_values(array_map('strtolower', $header));
+
+        foreach (['pcd', 'lat', 'long'] as $required) {
+            if (($headerCounts[$required] ?? 0) > 1) {
+                $this->error("Column '{$required}' appears more than once — refusing to import.");
+                fclose($handle);
+
+                return self::FAILURE;
+            }
+        }
+
         $columns = array_change_key_case(array_flip($header), CASE_LOWER);
 
         foreach (['pcd', 'lat', 'long'] as $required) {

tests/Feature/Console/ImportPostcodesTest.php

@@ -5,6 +5,7 @@ use Illuminate\Foundation\Testing\RefreshDatabase;
 
 uses(RefreshDatabase::class);
 
+if (! function_exists('writeOnspdFixture')) {
     function writeOnspdFixture(string $contents): string
     {
         $path = tempnam(sys_get_temp_dir(), 'onspd_').'.csv';
@@ -12,6 +13,7 @@ function writeOnspdFixture(string $contents): string
 
         return $path;
     }
+}
 
 it('imports active postcodes from an ONSPD CSV', function (): void {
     $csv = <<<'CSV'