List Upload de-duped against Lead Database

It's frustrating that uploading a list creates new leads. It would be nice if Marketo checked email addresses in the list against those in the database and added those leads from the database to the list, rather than creating a brand new record.

This would be helpful for instances when a list can't be pulled another way. For example, if there's a list of tradeshow attendees, but some of them are already in Marketo, you'd just want to upload the list and not worry about duplicates.