I use Lightroom 6. In the library tere is a collection named "Duplicate Photo's" In there, there are several smart collections (like: "import-oldest"). I dit not create this (smart) collection and I don't know what is does.
It uses "Duplicate Id" as criterion. What does that mean?
Smart Collections are only available in Lightroom Classic whereas this forum is intended for question on Lightroom Desktop. Nevertheless, you've asked it here. So, best answered here rather than moving or reposting.
Since there is no built-in criterion called 'Duplicate ID'. Therefore, I suspect it's a custom criterion that you've created yourself or imported at some point.
Basically, the smart collection is looking for duplicate images using metadta not normally visible to the user. If you don't use it and don't know where it came from, then just delete it.