How to generate a random, non-repeating string C++ [duplicate]

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• How to shuffle a std::vector? 4 answers

I need to generate a code of specified length, and it has to be random and non repetitive, so far, I have the random string but it's repeating the letters

Here is my code;

#include <string>
#include <cstdlib>
#include <ctime>

class random_text_generator
{
public:
random_text_generator(const std::string& str = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789")
: m_str(str)
{
    std::srand(std::time(0));
}   
std::string operator ()(std::size_t len = 1)
{
    std::string seq;
    std::size_t siz = m_str.size();
    if(siz)
        while(len--)
            seq.push_back(m_str[rand() % siz]);
    return seq;
}
private:        
std::string m_str;
};

#include <iostream>

int main(void)
{
using namespace std;
random_text_generator rtg;
for(size_t cnt = 0; cnt < 7; ++cnt)
  {
    cout << rtg(cnt) << endl;
  }
}

Again, this code generates the string but it repeats some letters, for example the output would look like this

**OUTPUT**
A K X K Y

See that the K is repeating.