My Sim has a relationship to the same Sim twice (in the relationship panel twice)
Unclear. Probably has something to do with cleanups that are not properly done after Sims have been on vacation (WA and GEN Opportunity) or boarding school. Also inviting Uni sims to your graduation can lead to this issue.
Such double relationships generate two Sims with the same ID and that can lead to a lot of problems, for example Sims going missing from the portrait panel.