WFFM Error - An Item with the same key has already been added

Question

On my local environment all WFFM forms are working ok, however on our test environment when any form is submitted we get the error:

An item with the same key has already been added.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.ArgumentException: An item with the same key has already been added.

Source Error:

Line 24:     if (!IsPost)
Line 25:     {
Line 26:       queryString.Add("wffm." + Constants.FormItemId, Model.Item.ID.ToGuid());
Line 27:       queryString.Add("wffm." + Constants.Id, Model.UniqueId);
Line 28:     }

Source File: D:\home\site\wwwroot\Views\Form\EditorTemplates\FormViewModel.cshtml Line: 26

Note: I have tried uncehcking the field on the form Is Ajax Mvc Form and this seems to resolve the issue.

Is there some configuration setting which could be causing this, since there is no difference in the form content itself between the two environments.

Answer 1

The issue turned out to be due to a custom 404 error handler (Sitecore Climber was on the right track when he mentioned about the IIS rewrite module - redirects can mess up WFFM!). It was intercepting requests (with a httpRequestBegin pipeline handler) and returning the 404 page content. The solution was to change the 404 handler to ignore requests where the URL started with /form.

Note: if you create a 404 handler, make sure it can ignore certain paths where a sitecore item is not resolved, otherwise you will break lots of things in Sitecore!