title: Join 2 columns with list values in pandas(Avoiding duplicates and NaN)
body: <p>I have pandas dataframe as follows,</p>
  <pre><code>loc_1                               loc_2                             
  
  [mumbai, gujarat, sri lanka]        [chennai, UP]
  [Goa, telangana]                    [Kashmir, Goa, Rajkot]
  NaN                                 [Bihar, Orissa]
  
  </code></pre>
  <p>I want to create a new column that is a combination of both the above columns,
  I did search other similar questions but the issue I am facing is that,</p>
  <p>When I do,</p>
  <pre><code>data['locations'] = data['loc_1'] + data['loc_2']
  
  Output
  --------
  loc_1                               loc_2                       locations
  
  [mumbai, gujarat, sri lanka]        [chennai, UP]                [mumbai, gujarat, sri lanka,chennai, UP]   
  [Goa, telangana]                    [Kashmir, Goa, Rajkot]       [Goa, telangana,Kashmir, Goa, Rajkot]
  NaN                                 [Bihar, Orissa]              NaN
  </code></pre>
  <h2>Issue</h2>
  <p>As you can see above, there are duplicate values as well as NaN values formed.
  How to avoid them?</p>
  <h2>Remember</h2>
  <p>The original dataset contains values in list, str and NaN format.</p>
  <h2>Dataset:</h2>
  <pre><code>
  loc = pd.DataFrame({
  'loc_1': [['mumbai', 'gujarat', 'sri lanka'],['Goa', 'telangana'],np.nan],
  'loc_2':[['chennai','UP'],['kashmir','goa','rajkot'],['bihar','orissa']],
  'loc_3':['Chennai','Bangalore','Vizag']
  
  })
  </code></pre>