When attempting to set NPCs up on dates, they'll no longer report back that the other sim is already in a relationship when that isn't the case. They'll also gain the proper relationship when they say they've started dating.
If you've never seen this before, the interaction is under Relationship, Other Sims (used to be Friendly, Gossip, Other Sims). Select Hook Up With and then you can choose who they should date. This always results in a call saying that the other sim is already seeing someone whether or not that is the case. Sometimes they'll call again saying they've started dating, but the relationship doesn't actually change. This mod fixes both of those issues so you can play matchmaker with your sims.