An item with the same key has already been added Sitecore EXM

If you are using Sitecore EXM campaign on Form submit button, then you may be facing below issue on UI screen as 

Send Email Failed...

and if you will see Sitecore EXM log will get that error information as

Message: an item with the same key has already been added.


To overcome with this issue, would suggest to go back on the Form Builder and ensure that all form Fields name are unique, and if not then please update will started working.

Hope above tips will help to resolve this issue.