IIRC, there will only be one instance of a specific scheduled search running at any given time. I see this happening, also, when I schedule a search to happen before the previous run finishes. For example, a search that takes 3 minutes to complete is scheduled to run every minute. I will have two schedule searches fail to launch before the first one completes, then on the fourth minute, the search fires correctly again, followed by two more launch fails. You can get around this by creating a different named search to run while the backfill script is operating. They could do the same things, but they have to be different search names.
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