Overlap happens when calls are assigned to whoever is free instead of whoever is closest. It's invisible on a daily board and obvious on a map.
Rule 1: One owner per zip
Give each tech a home territory. Calls in that zip go to them first.
Rule 2: Batch maintenance
Cluster tune-ups and agreement visits by zip and day. Tuesdays in one area, Wednesdays in the next.
Rule 3: Look back weekly
Upload last week's jobs and check the overlap count. If it's not going down, the rules aren't being followed.