//Convert the string into an int
num =  Integer.parseInt(inputField.getText());

// Add it to the an index
array[index] = num;
// Increment the index variable
index++;
// If the the duplicate exists
for(int i = 0; i < array.length;i++){
    if(array[index] == num){
        if(array[i - 1] == num){
            JOptionPane.showMessageDialog(null,"Array may not contain duplicates ","Array Duplicate",JOptionPane.ERROR_MESSAGE );
            break;
        }
    }
}