Uploading a list of contacts from an event without creating dupes

This shouldn't be this difficult, but no one can seem to give me a straightforward process.

1. I have a list of contacts from an event.
2. Some of these are already in Dynamics, some are not.
3. I want to import them into Dynamics. I don't want to create dupes, but I DO want to retain them connected as a list.

It seems like I can either import them all and create dupes, and use some unused field to keep them connected. Or

I can only import the new ones, and then the existing ones are unlinked.

It seems that somehow the marketing list functionality must be connected to this.

What am I missing?