Create a new string made of 2 copies of the first 2 characters of a given string

import Foundation
func two_copies(_ str1: String) -> String {
    
    if str1.characters.count == 0 {
        return ""
    } 
    else if str1.characters.count == 1 
    {
        return str1 + str1
    } 
    else 
    {
        var currentIndex = str1.index(after: str1.startIndex)
        currentIndex = str1.index(after: currentIndex)
        let res = str1.substring(to: currentIndex)
        return res + res 
    }
    
}

print(two_copies("Swift"))
print(two_copies("Online"))
print(two_copies("JS"))

Output:

SwSw
OnOn
JSJS