function findFirstRepeatedChar(s){
  for(let i=0; i<s.length; i++){
    if(s[i] == s[i+1]){
      return s[i];
    }else if(s.indexOf(s[i], i+1) != -1){
      return s[i];
    }
  }
  return false;
}
console.log(findFirstRepeatedChar("ABCCBD"));
//console.log(findFirstRepeatedChar("ABCDB"));
//console.log(findFirstRepeatedChar("ABCDE"));