You need some code to do selection-without-replacement so you don't choose the same agent multiple times. There's nothing built-in to AnyLogic to do this for you, so you'd want code something like the below (assuming your agents are of type Person).
// Create selection list with all Person agents in
List<Person> selectionList = new ArrayList<Person>();
for (Person p : people) {
selectionList.add(p);
}
// Select one 800 times (removing them from our selection list as we go)
for (int i = 0; i < 800; i++) {
int randomIndex = uniform_discr(0, selectionList.size() - 1);
send ("s1 to s2", selectionList.get(randomIndex));
selectionList.remove(randomIndex);
}
The crucial thing to remember (and something that many AnyLogic modellers don't understand) is that messages to statecharts are processed asynchronously (in hidden timeout 0 events which you can see on the Events View at runtime) however you send them (e.g., whether you use send, receive or the statechart's fireEvent function --- these just affect the number of such intermediate events and whether the "On message received" action of the Connections element is triggered or not).
Thus an agent will never have changed state in a block of code which sends messages to them. (It will only do so after this block of code --- and the current event it is triggered from --- completes.)
P.S. As per your comment to Emile's (incorrect) answer, the other way to do this is via dynamic events so that each message is sent after previous ones have been processed. But then you have to be very careful that you get the at-the-same-sim-time event ordering correct (loads of subtle detail here) and you'd still have to do the filtering in Emile's answer which is very wasteful; much easier to do it the more conceptually correct way where you whittle down the set of agents you're sampling from as you go.
