title: Is there any problem with pandas merge currently?
body: <p>Using outer join to merge two tables. Let's say </p>
  
  <p><code>df1 = ['productID', 'Name']</code></p>
  
  <p><code>df2 = ['userID', 'productID', 'usage']</code></p>
  
  <p>I tried to use outer join with merge function in pandas. </p>
  
  <p>pd.merge(df1, df2[['userID','productID', 'usage']], on='productID', how = 'outer')</p>
  
  <p>However, the error message I got is 
  <code>'productID' is both an index level and a column label, which is ambiguous.</code></p>
  
  <p>I googled this error message and saw a open [issue]: <a href="https://github.com/facebook/prophet/issues/891" rel="nofollow noreferrer">https://github.com/facebook/prophet/issues/891</a> </p>
  
  <p>Any solution to my problem?</p>