Prevent duplicate/overwritten keys

src/Tree/ObjectNode.php

@@ -15,8 +15,16 @@ class ObjectNode extends Node implements CollapsibleNode
         $this->properties[$key] = $value;
     }
 
+    public function hasKey(string $key): bool
+    {
+        return array_key_exists($key, $this->properties);
+    }
+
     public function rename(string $key, string $newKey)
     {
+        if (array_key_exists($newKey, $this->properties)) {
+            throw new \Exception("Key {$newKey} already exists");
+        }
         $renamed = [];
         foreach ($this->properties as $k=>$v) {
             $renamed[($k==$key)?$newKey:$k] = $v;