How to fill an int array with random values from a range with exactly one duplicate for each value?

I am trying to create a memory match game using JFrame. I am using a folder that contains .png files (country flags) which will populate a String array using the following code:

public String[] listPngFiles(String filePath) {
    File[] fileReader = fileReader(filePath);
    String[] files = new String[fileReader.length];

    for(int i=0; i<files.length; i++)
        files[i] = "" + fileReader[i];

    arraySize = files.length;
    return files;
}

public File[] fileReader(String filePath) {
    File folder = new File(filePath);
    return folder.listFiles (new FilenameFilter() { 
        public boolean accept(File filePath, String filename)
        { return filename.endsWith(".png"); }
    });
}

The reason I am doing is because I want to create a dynamic JButton array that holds each flag in a random sequence, twice. The flags have to re-shuffled every time the program is executed.

This is the code I am currently working on:

int[] flags = new int[arraySize*2];

for (int i=0; i<flags.length; i++){
    flags[i] = -1; // -1 represents "no flag"

    for (int r=0; r<2; r++)
    {
        int pos=0;
        do
        {
            pos = randomize();
            flags[pos] = i; // put the flag into the card at the available position
            }
            while (flags[pos] != -1); // if card[pos] is not -1, it already has a flag in it
        }
    }

Your help on the matter is greatly appreciated.