Proper usage of std::atomic, pre increment value as function param - c++

Question

I have code where I need unique id (packet id for some protocol). So I used std::atomic<int>. After reading documentation I was confused because it stated that increment is done in this way. fetch_add(1)+1
I understand that value inside fetch_add is incremented atomically but I get pre-increment value +1 outside atomic operation. What I would guess is not atomic.
Can I use some_func(++atomic_value)?
I wrote simple code to check if it works. And it works but I don't understand why.
#include<iostream>
#include <atomic>
#include <thread>
#include <vector>
#include <random>
#include <mutex>
#include <algorithm>
std::atomic<int> Index = 0;
//int Index = 0; // non atomic Index. It will generate duplicities
std::vector<int> Numbers;
std::mutex Mutex;
std::default_random_engine Generator;
std::uniform_int_distribution<int> Distribution(5, 10);
void func(int Value)
{
std::lock_guard<std::mutex> Guard(Mutex);
Numbers.push_back(Value);
}
void ThreadProc()
{
Sleep(Distribution(Generator));
func(++Index); // is this proper usage of std::atomic?
}
int main()
{
const int ThreadCount = 1000;
std::vector<std::thread> Threads;
for ( int i = 0; i < ThreadCount; i++ )
{
Threads.push_back(std::thread(ThreadProc));
}
for_each(Threads.begin(), Threads.end(), [](std::thread& t) { t.join(); });
std::sort(Numbers.begin(), Numbers.end());
auto End = std::unique(Numbers.begin(), Numbers.end());
if ( Numbers.end() == End )
{
std::cout << "No duplicites found." << std::endl;
}
else
{
std::cout << "Duplicites found ! - " << Numbers.end() - End << std::endl;
for_each(End, Numbers.end(), [](int n) { std::cout << n << ", "; });
}
return 0;
}
Off-topic question: When I defined Index as non atomic I get duplicities but only from end of range. Numbers are always 900+. Why it is so?