diff --git a/src/File.php b/src/File.php
index 0fc71d12..573416e2 100644
--- a/src/File.php
+++ b/src/File.php
@@ -1479,6 +1479,11 @@ public static function getAllowedExtensions()
                 continue;
             }
 
+            // don't add the same extension multiple times
+            if (in_array($key, $allowedExtensions)) {
+                continue;
+            }
+
             $allowedExtensions[] = $key;
         }
         return $allowedExtensions;