title: How can I find elements appear twice (or other specific times) in a list
body: <p>As title, I have a list and I want to find the element that appear twice.</p>
  
  <p>For instance, a = [1,2,3,3,4] and I want to get "3".</p>
  
  <p>I came up with a for loop using list.count(element)==2 to get the number but it's quit time wasting. Is there other ways without using for loop? </p>
  
  <p>thanks a lot</p>