An alternative, `scan`-based implementation of `Publisher.removeKnownDuplicates`

scan_remove_known_duplicates.swift

import Combine

extension Publisher where Output: Hashable {
	func removeKnownDuplicates() -> Publishers.CompactMap<Publishers.Scan<Self, (Set<Output>, Output?)>, Output> {
		scan((Set<Output>(), Output?.none)) { seenAndPrevious, incoming in
			var seen = seenAndPrevious.0
			let isIncomingUnique = seen.insert(incoming).inserted
			return (seen, isIncomingUnique ? incoming : nil)
		}
		.compactMap(\.1)
	}
}