This error means you have a deleted run named the same thing. If you have two runs in your database named the same thing, KC will duplicate those appointments on the schedule.
Click on Setup, Lists, Runs.
Click on "Show runs marked as deleted". All runs should be in red for Deleted.
Find the run with the same name you want to add. Click on it and hit Delete Run.
You may will get a popup that no appointments will be shown for this run. Click Yes. This is already a deleted run. You are permanently deleting it from your database.
Now you are ready to add the run. Click on Show active runs and hit Add New Item.