We get a lot of requests via email that are already a feature in the system, but we still want to track the request. It is a pain to have to download the email and save it as an attachment on the feature.
It's currently possible to create ideas via email. You can find the email address by going to Settings->Product->Configure ideas.
The best way to handle this use case would be to create an idea from the email. You can then promote the idea to the existing feature of the item which already exists in the system.
I still think this a valid request. The Idea route still requires several steps, and potentially can skew some metrics that might be tracked on what "real" ideas are promoted to a Feature. Would be interested to see this reconsidered.