Swift currently crashes at runtime with "Fatal error: "Dictionary literal contains duplicate keys" if you have duplicate keys in a dictionary literal. They even don't print actual duplicate keys. This struct collects all the duplicate keys and print them with count.

DuplicateKeyChecker.swift

struct DuplicateKeyChecker<Key: Hashable, Value>: ExpressibleByDictionaryLiteral {
    init(dictionaryLiteral elements: (Key, Value)...) {
        var count: [Key: Int] = [:]
        for (key, _) in elements {
            count[key] = (count[key] ?? 0) + 1
        }
        print(#function, "duplicate keys:", count.filter { $0.value > 1 })
    }
}

// usage: specify the type of variable as DuplicateKeyChecker
let test: DuplicateKeyChecker = ["foo": 1, "bar": 2, "foo": 3,]

// prints this
init(dictionaryLiteral:) duplicate keys: ["foo": 2]