Effective dating: links that stop being true — Overlaps are refused by default

Writing a period that overlaps an existing one on the same link is rejected unless you say otherwise. A record quietly having two parents on the same day is the kind of wrong that only ever surfaces when one report disagrees with another. Where overlap is genuinely real — someone who honestly did report to two managers — you can soften the policy on that field to warn (allowed, but flagged) or allow (silent).