To avoid the behavior @monomesa said where people will try to use “already known” for everyone and hope some people do the same, we can give a penalty to a person who chooses “already known” when someone else chooses “just met.”
We could subtract mismatched levels from matching levels–so if you had 10 connections where you chose “already known” and the other person chose “just met” and 10 where you both chose “already known” they would cancel each other out and you would have 0 qualifying “already known” connections.
We could record someone’s initial confidence level choice for the purpose of detecting mismatches, since we allow people to change the confidence level. (We could store both current and initial level).
If we let people know there’s a penalty for a mismatched choice, it will stop 99% of people from naively trying it.