title: SQL Join in R: duplicate column error WITHOUT a duplicate column
body: <p>I am trying to do a simple merge of two datasets using sqldf in R. The code line is as follows:</p>
  <pre><code>data_complete &lt;- sqldf(&quot;SELECT df1.*, df2.*
                      FROM df1 LEFT JOIN df2 
                      USING(ccn_number)&quot;)
  </code></pre>
  <p>I get the following error message: &quot;Error: duplicate column name: zip5&quot;</p>
  <p>However, the variable zip5 is ONLY in df1, not df2, because I had already removed it from the latter, so there is no duplicate column name. I have tried some variations on the syntax, but continue getting the same error. I even tried deleting zip5 from a CSV version of df2 and then importing that, but still got the error. Any advice would be greatly appreciated, and thank you!</p>