This really isn't a new idea. It's a feature in the latest version of Act! Premium. It's just that it doesn't work.
While importing from an Excel spreadsheet, I chose custom merge so I could confirm each match. Upon finding a duplicate in the very first record being imported, I chose to "skip" rather than create new contact or merge the information. Instead of going to the next duplicate found, it just terminated the import.
When I first reported this bug in May, 2013, here's the comment I received from Act! Support:
"I have tested and reproduced this same effect, and have submitted it to the Development team for investigation. Thank you for bringing this to our attention!"
And they still haven't fixed it!
This is a serious flaw in the program. It's not just a desired feature, but a feature that exists and doesn't work. Someone needs to escalate this. Without it, importing records can't be automated because of the risk of duplicates.