iOS Swift 2 - Check if function already in array

Question

This is my array for function

var funcArray = Array<() -> Void>()

To the array I append closure functions which comes as a parameter

func someFunc(completition: (() -> Void)?){
// ...
     if let callback = completition {
         funcArray.append(callback)
     }
// ...
}

How can I check if the array already contains the function before appending it to the array ?