The trigger activates when an indep unit enters its area or spawns in. This trigger must be deactivated before it runs again (rearmed).
So, your first unit/group must leave the area or must be treated for exiting thisList (need a variable for example).
What I suggest:
1. If you know how to apply a code on spawned unit/group by your spawned module, add your waypoint this way (no trigger). I don't know if these units/groups are identified somewhere.
2. Far simpler : use MGI advanced modules, especially spawn Groups attack which allows you any group(s), of any side, of any mod, and any behavior (not only attack)... and code straight in module (here just write : (_this #0) addWaypoint [getMarkerPos "TaskAssault_Mark",0]; in code for group(s) field.) Simple as that.
3. Try to hack your spawned group. Name your trigger (the area one, say: spawningZone)
addMissionEventHandler ["GroupCreated", {
params ["_grp"];
_grp spawn {
params ["_grp"];
sleep 0.5;
if (side _grp == INDEPENDENT && units _grp inAreaArray spawningZone isNotEqualTo []) then {
_grp addWaypoint [getMarkerPos "TaskAssault_Mark",0]
};
};
}];