diff --git a/app/Console/Commands/ImportPostcodes.php b/app/Console/Commands/ImportPostcodes.php
index 5317b62..c4fdd90 100644
--- a/app/Console/Commands/ImportPostcodes.php
+++ b/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) {
diff --git a/tests/Feature/Console/ImportPostcodesTest.php b/tests/Feature/Console/ImportPostcodesTest.php
index 0e9b076..bd43cfb 100644
--- a/tests/Feature/Console/ImportPostcodesTest.php
+++ b/tests/Feature/Console/ImportPostcodesTest.php
@@ -5,12 +5,14 @@ use Illuminate\Foundation\Testing\RefreshDatabase;
 
 uses(RefreshDatabase::class);
 
-function writeOnspdFixture(string $contents): string
-{
-    $path = tempnam(sys_get_temp_dir(), 'onspd_').'.csv';
-    file_put_contents($path, $contents);
+if (! function_exists('writeOnspdFixture')) {
+    function writeOnspdFixture(string $contents): string
+    {
+        $path = tempnam(sys_get_temp_dir(), 'onspd_').'.csv';
+        file_put_contents($path, $contents);
 
-    return $path;
+        return $path;
+    }
 }
 
 it('imports active postcodes from an ONSPD CSV', function (): void {