Generate random String without repeating in swift

I want the function to generate random String without repeating. For example this function maybe will print: ABCC

func randomString(length:Int) -> String {
let charSet = "ABCDEF"
var c = charSet.characters.map { String($0) }
var s:String = ""
for _ in (1...length) {
    s.append(c[Int(arc4random()) % c.count])
}
return s
} print(randomString(length: 4))

and i want print random unique string only, E.g : ABCD