Check out example codes for "java check if string appears twice in arraylist". It will help you in understanding the concepts better.

Code Example 1

//same as the Grepper awnser but for anything
//gets the amount of times the object appears in the array
public static int amountOfTimesObjectAppearsInArray(ArrayList<?> array, Object checkMe) {
    int numCount = 0;
    for (Object o : array) {
        if (o.equals(checkMe)) numCount++;
    }
    return numCount;
}
//check if it appears more than once
public static boolean objectAppearsInArrayMoreThanOnce(ArrayList<?> array, Object checkMe) {
    return amountOfTimesObjectAppearsInArray(array, checkMe)>1;
}

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