That sounds like exactly the feature we were hoping for. I'm on v10 and it's apparently called "Enable duplicate address check...". We can look to upgrade to v11 where it's called "Filter duplicate incoming emails" - but has this feature changed between versions?
The current naming "...duplicate address check..." makes it sound like the same address can't send more than one ticket, which we obviously wouldn't want turned on.
Also, I didn't see details on exactly how this feature works - does it look for duplicates within the same import batch? Or does it compare the current import to the contents of a previous import and filter out duplicate emails across import batches?