Currently when the ExternalDataSources name isn't unique the backend sends a response like so
"org.springframework.context.NoSuchMessageException: No message found under code 'unique'
There should actually be a better message than that. It needs adding to the message bundle in the backend. Its a missing translation key.
Update: Message should be the same as the toast message defined in
"External data source was not saved. Name must be unique. A data source with the name "dummy3" already exists."