How to ensure the same element doesn't appear twice in an array

Use Set instead of array for processing, to ensure it doesn't appear twice

Set<String> fileNames = new HashSet<String>();
fileNames.add("1.txt");
fileNames.add("2.txt");
// not necessarily in that order with HashSet
System.out.println(fileNames); //[1.txt,2.txt]
fileNames.add("1.txt");// it will not add this one
System.out.println(fileNames); //[1.txt,2.txt]

Loop through the array each time you add the item, and compare the two. If they are the same, don't add the new one.

    for (int i = 0; i < array.length; i++)

    if (object = array[i]) {
       //DONT ADD THE OBJECT HERE
    else {
        //add the object
    }

}

Using a set is the best option. Another way can be to use a HashMap with file name as key and file object as value.

HashMap<String,File> fileMap = new HashMap<String,File> ();

if (fileMap.get("fileName")==null)
fileMap.put("fileName",file)

In Kotlin :-

fun singleNumber(nums: IntArray): Int {
        val set=hashSetOf<Int>()
        for(x in nums){
            if(set.contains(x)){
                set.remove(x)
            }else{
                set.add(x)
            }
        }
        return set.first()
    }