Fixes T10145. I went with "don't add two panels", since panels are easy to create, I expect this to be a reasonable limit until we have better use cases.
- Maniphest Tasks
- T10145: Adding a panel to a dashboard multiple times
- rP5556f0e45a52: Don't allow duplicate panels on dashboards
Try to add the same panel twice, get error. Add panel normally fine, move panels fine, edit panels fine.