The following is function in c++, Please help me with the time complexity Big-O and please explain your answer:

bool foundTwice(string &s)

{

for(int i = 0; i < s.length(); i++) {

for(int j = i+1; j < s.length(); j++) {

if(s[i] == s[j]) {

return true;

}

}

}

return false;

}