When storing a Web Form, the error, "An Item with the same key has already been added" appears.

This is caused by a duplicate Field ID being present in the form when created. Normally, this is checked for and prevented before publishing the form, but when it does occur it prevents users from storing new forms. If it does occur, check all fields for duplicate IDs.

You can confirm the Field IDs by opening up the Form experiencing the problem in configuration. You will then be able to click each field and the Field ID will be displayed on the right-hand side. Once all fields are confirmed to not be duplicated, the error will be resolved.