But now I have another problem. How do I update the duplicate record that doesn't get updated by the list import?
Well... you can't. Unless you use the API. Because an import will only ever update a single lead per row, and UI-based imports dedupe on the Email Address field. An API-based import can dedupe on ID (though you'd have to the different IDs in advance, which could be done with additional API work). A webhook-driven process could also update all leads with a matching email address, but wouldn't be advised in large quantities (let's say "large" = > 10,000 leads triggered at the same time).
... View more