string s,t;
int main(){
    cin>>s;
    bgn:
    bool b=0;
    s=s+'_';
    Repi(s.sz-1){
        if(s[i]==s[i+1]){i++;b=1;}
        else t=t+s[i];
    }
    s=t;t="";
    if(b==1)goto bgn;
    cout<<s<<endl;
    system("pause");
    return 0;
}

Any idea where's the problem?

Try "aaa".
Answer is "".