A duplicate subscription in Email Autoresponder is defined as multiple subscriptions to one autoresponder by the same email address.
The De-Duplicate function quickly removes duplicate subscriptions. The first subscription remains - all others are permanently deleted.
To de-duplicate your subscriber list, select Subscribers » Global Tasks » De-Duplicate from the menu bar. You will be asked to confirm your choice of action.
This action is a global action. It ignores any filter you have in place.