An initial sketch of Publisher.pairwiseWithDuplicatedStart.

pairwise_with_duplicated_start_sketch.swift

import CombineExt

let upstream = (1...2)
	.publisher
	.share()

let cancellable = upstream
	.first()
	.flatMap { first in
		upstream
			.prepend(first, first)
	}
	.pairwise()
	.sink(receiveValue: { print($0) })

// Outputs:
// ```
// (1, 1)
// (1, 1)
// (1, 2)
// ```