title: add new item into array after verifying that it does not exist already
body: <p>I want to insert a new item into an embedded array  , but before that I need to make sure that that item does not exist already in the array : 
   thats the collection </p>
  
  <pre><code>"Accounts":[
  {
      "Account_id" : 70,
      "FirstName" : "name",
      "LastName" : "bffff",
      "Username" : "deee",
      "Password" : "xyzqds@j",
      "AccountType" : "admin",
      "Created_at" : "01-01-2018",
      "Rules" : {
          "Goal" : 480,
          "DoNotDisturbFrom" : "22",
          "DoNotDisturbTo" : "8",
          "Frequency" : "weekly"
      }
  },
  {
      "Account_id" : 52,
      "FirstName" : "joe",
      "LastName" : "Doe",
      "Username" : "aajengui1",
      "Password" : "abc@j",
      "AccountType" : "user",
      "Created_at" : "01-02-2018",
      "Rules" : {
          "Goal" : 260,
          "DoNotDisturbFrom" : "10",
          "DoNotDisturbTo" : "12",
          "Frequency" : "monthly"
      }
  }
  </code></pre>
  
  <p>]</p>
  
  <p>I want to append a new account into the array but it has to be unique so it's like verifying that the username / email does not exist already </p>