title: Merge columns into one while dropping nan values and duplicates
body: <p>I am trying to merge multiple columns into a single column while dropping duplicates and dropping null values but keeping the rows.
  What I have:</p>
  <pre><code>df= pd.DataFrame(np.array([['nan', 'nan', 'nan'], ['nan', 2, 2], ['nan', 'x', 'nan']]), columns=['a', 'b', 'c'])
  </code></pre>
  <p>What I need:</p>
  <pre><code>df= pd.DataFrame(np.array([[''], [ 2], [ 1]]), columns=['a'])
  </code></pre>
  <p>I have tried this but I get 1,nan for the last row:</p>
  <pre><code>df['a]=df[['a','b','c]].agg(', '.join, axis=1) 
  </code></pre>
  <p>I have also tried the following but I cannot get this to work:</p>
  <pre><code>.stack().unstack() 
  </code></pre>
  <p>and</p>
  <pre><code>.join 
  </code></pre>
  <p>but I cannot get these to drop duplicates for each row</p>