String str1 = "ABCDABCD";
String result1 = "";

for (int a = 0; a <= str1.length()-1; a++) {
if (result1.contains("" + str1.charAt(a))) { 
// charAt methodda you provide index number ve sana character olarak donuyor,
// If the string result does not contains str.CharAt(i), 
// then we concate it to the result. if it does we will not
   continue;
}
result1 += str1.charAt(a);
}
System.out.println(result1);