Q: what happens if I run the exchange rebasing tool twice on a group of users?
A: When you run the tool, it will stamp all the appointments that were rebased with the required information so that a second run doesn't affect them.

One thing to remember is if you run the rebasing today and you run it again in 1 week, the single instance appointments that were created during that week, will be rebased incorrectly even though they were created with the right DST rules applied on the workstation.