Maybe an overkill for your situation... In my use case see Use of NodeRed for international weather, climate, ocean, hydrology data exchange I am using redis to do this kind of check. We use the payload j3hh432j as a key in redis. Through redis get/set it is fairly easy to do this. Obviously, you'll need a redis instance (typically a docker version of it will do) for this to work.
As Colin wrote, an array in a persistent context is simpler than "grepping" into a file...
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