How can I detect potential duplicates in a CSV I want to import?
I like Allen's answer but it seems there should be an approach at the point of import that would work. Esp if you had a gazillion contacts in the db already. Is the issue here is that the potential matches are more complex than any matching rules?

If there is a Matching Rule or a series of them that can work for you - then start with the loosest rule and then rerun and tighten them - and set your import to SKIP.

Eg, if you first run the import with First Name match, then every record that has a matching first name will be reported back to you. Then run it with eg First AND Last, or First AND Phone etc.