title: Validate in merge function pandas
body: <p>Today I was trying to go a little deeper in the <code>merge()</code> function of pandas, and I found the option <code>validate</code>, which, as reported in the documentation, can be:</p>
  <blockquote>
  <p>validate : string, default None</p>
  <p>If specified, checks if merge is of specified type.</p>
  <p>“one_to_one” or “1:1”: check if merge keys are unique in both left and
  right datasets. “one_to_many” or “1:m”: check if merge keys are unique
  in left dataset. “many_to_one” or “m:1”: check if merge keys are
  unique in right dataset.   &quot;many_to_many” or “m:m”: allowed, but does
  not result in checks.</p>
  </blockquote>
  <p>I have looked around to find a working example on where and how to use this function, but I couldn't find any. Moreover when I tried to apply it to a group of <code>DataFrame</code>'s I was merging, it didn't seem to change the output.
  Can anyone give me a working example, to make me understand it better?</p>
  <p>Thanks in advance,</p>
  <p>Mattia</p>