Add ability to disable redundant scheduled pipelines

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Description

The scheduled pipelines feature that was added a couple releases ago is useful. However, it can be wasteful, as it will always trigger a pipeline at the scheduled time, even if no additional commits have been made since the last pipeline. I propose adding an option to the scheduled pipeline configuration that will suppress the generation of pipelines that are redundant (i.e. if a pipeline already exists for that commit).

Documentation blurb

Overview

Use cases

Anyone who cares about resource usage on their runners. This can reduce costs if the pipelines are being run via a cloud service, for example. If builds are determinstic (which in most cases they should be), then there is no advantage to creating a new pipeline for a commit that already has one.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.

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