can't find why this bug occur

[nj1234]

in my function i like to take a char from the user but that char must be small letter and same letter cannot be used twice

  #include <iostream>
#include <string>

using namespace std;
char letter(string &);
int main()
{   //dont look at this i just randomly tasting 
    string alo=" "; char a;
    for (int i=0;i<3;i++){ 
   a= letter(alo);}
    cout<<a<<" string "<<alo;

    return 0;
}
char letter(string &alo){
    char a;bool ok=true;
    
    while (ok){
        cin>>a;
        for (int i=0;i<alo.size();i++){
         if(a>='a'&&a<='z'&&a!=alo[i]){ //here a!=alo[i] is ignored 
             alo+=a;
           ok=false;
                       return a;
             
         }
         else 
         cout<<"bad input";
         
        
    }}
}

[AbstractionAnon]

Line 21: The first char of alo is a space, so a!=alo[i] is always going to be true the first time through the loop. That's always going to cause you to return a at line 24 the first time through the loop.