Task

#include <iostream>
#include <fstream>
#include <vector>
using namespace std;
int main()
{
    ifstream fin("IN.txt");
    ofstream fout("OUT.txt");
    if (!fin.is_open())
        cout << "File cannot be opened!\n";
    else
    {
        int kolvo;
        fin >> kolvo;
        if (kolvo <= 20)
        {
            float buff[20];
            for (int i = 0; i<kolvo; ++i)
                fin >> buff[i];
            for (int i = 0; i < kolvo; i++)
                fout << buff[i] << " ";
            fout << endl;
            vector <float> vec_buff;
            for (int i = 0; i < kolvo; i++)
                for (int j = i + 1; j < kolvo; j++)
                    if (buff[i] == buff[j])
                        vec_buff.push_back(buff[i]);
            for (int i = 0; i < vec_buff.size(); i++)
                for (int j = i + 1; j < vec_buff.size(); j++)
                    if (vec_buff[i] == vec_buff[j])
                    {
                        vec_buff.erase(vec_buff.begin() + j);
                        j--;
                    }
            fout << vec_buff.size() << endl;
            fin.close();
            fout.close();
        }
        else
            cout << "Number of elements must be under 21!!!!";
    }
    return 0;
}