Update intersection.md

snippets/intersection.md

@@ -10,7 +10,7 @@ Create a `Set` from `b`, then use `Array.prototype.filter()` on `a` to only keep
 ```js
 const intersection = (a, b) => {
   const s = new Set(b);
-  return a.filter(x => s.has(x));
+  return [...new Set(a)].filter(x => s.has(x));
 };
 ```