Today, when you do a bulk upload of users, the upload fails once 10 duplicate email addresses are found. The upload should be redesigned to ignore existing users (or update any changed fields included in the bulk upload) rather than kill the upload.
With the current limitation, admins have to do a download of existing users from Aha!, bump that list against the source list for Ideas accounts (e.g. list of all product users), and then manually remove all the users who already have accounts before doing the bulk upload into Aha. Super tedious and time consuming. The bulk upload should be a 2-minute activity, especially with the other limitations on user management.