adding some comments to dedupe file

Author: Nataliia74

Sprint-1/implement/dedupe.js

@@ -1,11 +1,14 @@
 function dedupe(elements) {
   if (!Array.isArray(elements)) {
+    // checking if an input is an array
     return null;
   }
 
   let new_array = [];
   for (let i = 0; i < elements.length; i++) {
+    // checking if new_array include elements from an elements array
     if (!new_array.includes(elements[i])) {
+      // if not push them to new array
       new_array.push(elements[i]);
     }
   }