Scripts

I suspect that Biara also enters the region and triggers the second event, and you don't have time to disable it because of the sleep function. Try this:

function fb1()
Trigger(REGION_ENTER_AND_STOP_TRIGGER, "stfb1", "fb1", nil);
RemoveHeroCreatures("RedHeavenHero03", 8, 20);
sleep(1);
RemoveHeroCreatures("Biara", 20, 1*D);
sleep(1);
StartDialogScene("/Maps/SingleMissions/Warprolog/StartScene2/DialogScene.xdb#xpointer(/DialogScene)");
sleep(1);
MoveHeroRealTime("Biara", 88, 59, GROUND);
sleep(1);
end;

Trigger(REGION_ENTER_AND_STOP_TRIGGER, "stfb1", "fb1");