There have been customizations I have done for Data Collection (DC), both the 2.x/2009 and 3.x/2012 versions, which have exhibited a behavior such that the form data appears to have been submitted to DC twice. That is, the code to process the form has executed once (“posted”), and then immediately executed again. Sometimes this results in duplicate data being collected, and sometimes, based on how the form is coded, the data is recorded once and then an error is returned saying that the data was rejected for being a duplicate. After investigation, we realized this was a feature of Microsoft’s Internet Information Service — the web server that DC runs on. To prevent this, we have used a coding trick to identify this situation and avoid the duplicate posting.