Combine framework: remove duplicates by key paths.

RemoveDuplicates.swift

import Combine

@available(OSX 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0, *)
extension Publisher {
    func removeDuplicates<Property>(by keyPath: KeyPath<Output, Property>)
        -> Publishers.RemoveDuplicates<Self> where Property: Equatable
    {
        return self.removeDuplicates {
            $0[keyPath: keyPath] == $1[keyPath: keyPath]
        }
    }

    func removeDuplicates<Property0, Property1>(
        by keyPath0: KeyPath<Output, Property0>,
        _ keyPath1: KeyPath<Output, Property1>
    ) -> Publishers.RemoveDuplicates<Self> where
        Property0: Equatable, Property1: Equatable
    {
        return self.removeDuplicates {
            $0[keyPath: keyPath0] == $1[keyPath: keyPath0] &&
                $0[keyPath: keyPath1] == $1[keyPath: keyPath1]
        }
    }

    func removeDuplicates<Property0, Property1, Property2>(
        by keyPath0: KeyPath<Output, Property0>,
        _ keyPath1: KeyPath<Output, Property1>,
        _ keyPath2: KeyPath<Output, Property2>
    ) -> Publishers.RemoveDuplicates<Self> where
        Property0: Equatable, Property1: Equatable, Property2: Equatable
    {
        return self.removeDuplicates {
            $0[keyPath: keyPath0] == $1[keyPath: keyPath0] &&
                $0[keyPath: keyPath1] == $1[keyPath: keyPath1] &&
                $0[keyPath: keyPath2] == $1[keyPath: keyPath2]
        }
    }

    func removeDuplicates<Property0, Property1, Property2, Property3>(
        by keyPath0: KeyPath<Output, Property0>,
        _ keyPath1: KeyPath<Output, Property1>,
        _ keyPath2: KeyPath<Output, Property2>,
        _ keyPath3: KeyPath<Output, Property3>
    ) -> Publishers.RemoveDuplicates<Self> where
        Property0: Equatable, Property1: Equatable, Property2: Equatable, Property3: Equatable
    {
        return self.removeDuplicates {
            $0[keyPath: keyPath0] == $1[keyPath: keyPath0] &&
                $0[keyPath: keyPath1] == $1[keyPath: keyPath1] &&
                $0[keyPath: keyPath2] == $1[keyPath: keyPath2] &&
                $0[keyPath: keyPath3] == $1[keyPath: keyPath3]
        }
    }
}

// etc, maybe repeat for more properties.
// until Swift have variadic generic parameters

@dduan using variadic generic parameters available in Swift 5.9:

extension Publisher {
    func removeDuplicates<each K: Equatable>(
        by keyPaths: repeat (KeyPath<Output, each K>)
    ) -> Publishers.RemoveDuplicates<Self> {
        removeDuplicates { lhs, rhs in
            func equate<Key: Equatable>(_ keyPath: (KeyPath<Output, Key>)) -> Bool {
                lhs[keyPath: keyPath] == rhs[keyPath: keyPath]
            }

            var allSatisfy = true
            repeat equate(each keyPaths) ? () : (allSatisfy = false)
            return allSatisfy
        }
    }
}