# tryRemoveDuplicates(by:) Publishes only elements that don’t match the previous element, as evaluated by a provided error-throwing closure. ``` func tryRemoveDuplicates(by predicate: @escaping (Self.Output, Self.Output) throws -> Bool) -> Publishers.TryRemoveDuplicates ``` ## Parameters `predicate` A closure to evaluate whether two elements are equivalent, for purposes of filtering. Return `true` from this closure to indicate that the second element is a duplicate of the first. If this closure throws an error, the publisher terminates with the thrown error. ## Return Value A publisher that consumes — rather than publishes — duplicate elements. ## Discussion Use [`tryRemoveDuplicates(by:)`](/documentation/Combine/Publisher/tryRemoveDuplicates(by:)) to remove repeating elements from an upstream publisher based upon the evaluation of elements using an error-throwing closure you provide. If your closure throws an error, the publisher terminates with the error. In the example below, the closure provided to [`tryRemoveDuplicates(by:)`](/documentation/Combine/Publisher/tryRemoveDuplicates(by:)) returns `true` when two consecutive elements are equal, thereby filtering out `0`, `1`, `2`, and `3`. However, the closure throws an error when it encounters `4`. The publisher then terminates with this error. ``` struct BadValuesError: Error {} let numbers = [0, 0, 0, 0, 1, 2, 2, 3, 3, 3, 4, 4, 4, 4] cancellable = numbers.publisher .tryRemoveDuplicates { first, second -> Bool in if (first == 4 && second == 4) { throw BadValuesError() } return first == second } .sink( receiveCompletion: { print ("\($0)") }, receiveValue: { print ("\($0)", terminator: " ") } ) // Prints: "0 1 2 3 4 failure(BadValuesError()" ``` --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)