Make sure created ICAO in factory is unique otherwise tests fail

app/Database/factories/AirportFactory.php

@@ -2,18 +2,36 @@
 
 use Faker\Generator as Faker;
 
+/**
+ * Create an ICAO for use in the factory.
+ */
+if (!function_exists('createFactoryICAO')) {
+    function createFactoryICAO(): string
+    {
+        $characters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
+        $max = strlen($characters) - 1;
+        $string = '';
+        for ($i = 0; $i < 4; $i++) {
+            try {
+                $string .= $characters[random_int(0, $max)];
+            } catch (Exception $e) {
+            }
+        }
+
+        return $string;
+    }
+}
+
 /*
  * Add any number of airports. Don't really care if they're real or not
  */
 $factory->define(App\Models\Airport::class, function (Faker $faker) {
+    $used = [];
     return [
-        'id' => function () {
-            $characters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
-            $string = '';
-            $max = strlen($characters) - 1;
-            for ($i = 0; $i < 4; $i++) {
-                $string .= $characters[random_int(0, $max)];
-            }
+        'id' => function () use ($used) {
+            do {
+                $string = createFactoryICAO();
+            } while (in_array($string, $used, true));
 
             return $string;
         },