 <p>Roughly, Magento is creating an order collection for you and attempting to load all the records. This collection has a rule that only allows it to create one order object for each ID, so when your additional object is loaded an exception is thrown.</p> <p>The left joins could be the issue, but it's hard to say off the bat. Could you post a little detail on how you are making the API call? An incorrect join can often have this problem.</p> <hr> <p>EDIT:</p> <p>If you're using the default code, my first guess would be that there are erroneous records in the database, or that this is an upgraded Magento system which had a bad upgrade in the past. Try this on a clean copy of your EE version pointing to the same database. If the same problem occurs, you may need to spelunk in the database looking for the reason for the problematic data load. Since you already have the query, you may want to separate out parts of the query to see if some subquery is returning too much data.</p>