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

flat_map_remove_known_duplicates.swift

import Combine

extension Publisher where Output: Hashable {
	func removeKnownDuplicates() -> Publishers.FlatMap<Publishers.Sequence<[Output], Failure>, Self> {
		var seen = Set<Output>()

		return flatMap { incoming in
			[
				seen.insert(incoming).inserted ?
					incoming :
					nil
			]
			.compactMap { $0 }
			.publisher
			.setFailureType(to: Failure.self)
		}
	}
}