SFE a.k.a Sitecore Forms Extension was pretty much mandatory for our Forms in Sitecore 9.1.1.
It gave us the functionality which we didn’t get out of the box, such as file upload that can be sent as an attachment in mail and some other Sitecore Forms field types like Html Field & Recaptha.
You also get a new Submit Action that sends email via EXM dispatch.
When we submitted the form I got this error in the logs, even though the form was accepted:
An item with the same key has already been added
This was sort of a silent error. The form was submitted and you recieved a thank you message on the page. But no email was sent.
Ultimately this was because there was two fields in the Form that had the same name. There is no validation of this in the Form itself so if you don’t know about it beforehand you could end up with this error.
Just make sure you use unique names on all the fields.
Sitecore Tackled 