title: amazon redshift concurrent write results in inserted records, causing duplicates
body: <p>I have been trying to solve a problem where two concurrent updates on the same table are causing additional records to be created/inserted. Never experienced this in any other relational database, and nor would i. So i believe it's potential a quirk in redshifts architecture of distributing queries across multiple nodes, however cannot pinpoint or provide a real world example.</p>
  
  <p>Before these two updates are run, i insert new data into the table. The insert contains a daily snapshot that fills out one day of data, most columns have empty values ready for the updates to populate them.</p>
  
  <p>The updates are run concurrently, which are simple update sql's, updating their respective columns. If run individually i do not see additional records created and no duplication.</p>
  
  <p>The updates operate across the entire table, over 200 million records, however the duplication occurs only in the records that where populated recently(the new data for that days period.</p>
  
  <p>This is kind of a worry, as i would never assume an update would ever create new records, addition to the records created with the first insert. </p>
  
  <p>What is even more bizzare is that the duplicate records hold different data. </p>
  
  <p>I have checked to veryify that no other queries are running beyond the expected, by looking at redshifts query logs (stl_query).</p>