Prevent duplicate spawns for same spawn ID

If there is an alive instance of the creature upon creature spawn, skip
spawn entirely. If there are only dead instances, despawn them. A new dead
corpse in the creatures home position will be respawned.

Closes: #16462
(cherry picked from commit fe2a0fda4464effeb2fb655da84a0460328b1aeb)

2 files changed, 32 insertions, 2 deletions

diff --git a/src/server/game/Entities/Creature/Creature.cpp b/src/server/game/Entities/Creature/Creature.cpp
index d75d0ab0fd4..9a6e118351a 100644
--- a/src/server/game/Entities/Creature/Creature.cpp
+++ b/src/server/game/Entities/Creature/Creature.cpp
@@ -1318,8 +1318,38 @@ bool Creature::CreateFromProto(ObjectGuid::LowType guidlow, uint32 entry, Creatu
     return true;
 }
 
-bool Creature::LoadCreatureFromDB(ObjectGuid::LowType spawnId, Map* map, bool addToMap)
+bool Creature::LoadCreatureFromDB(ObjectGuid::LowType spawnId, Map* map, bool addToMap, bool allowDuplicate)
 {
+    if (!allowDuplicate)
+    {
+        // If an alive instance of this spawnId is already found, skip creation
+        // If only dead instance(s) exist, despawn them and spawn a new (maybe also dead) version
+        const auto creatureBounds = map->GetCreatureBySpawnIdStore().equal_range(spawnId);
+        std::vector <Creature*> despawnList;
+
+        if (creatureBounds.first != creatureBounds.second)
+        {
+            for (auto itr = creatureBounds.first; itr != creatureBounds.second; ++itr)
+            {
+                if (itr->second->IsAlive())
+                {
+                    TC_LOG_DEBUG("maps", "Would have spawned %u but %s already exists", spawnId, creatureBounds.first->second->GetGUID().ToString().c_str());
+                    return false;
+                }
+                else
+                {
+                    despawnList.push_back(itr->second);
+                    TC_LOG_DEBUG("maps", "Despawned dead instance of spawn %u (%s)", spawnId, itr->second->GetGUID().ToString().c_str());
+                }
+            }
+
+            for (Creature* despawnCreature : despawnList)
+            {
+                despawnCreature->AddObjectToRemoveList();
+            }
+        }
+    }
+
     CreatureData const* data = sObjectMgr->GetCreatureData(spawnId);
     if (!data)
     {
diff --git a/src/server/game/Entities/Creature/Creature.h b/src/server/game/Entities/Creature/Creature.h
index 54e472a56d3..9103f864e3d 100644
--- a/src/server/game/Entities/Creature/Creature.h
+++ b/src/server/game/Entities/Creature/Creature.h
@@ -568,7 +568,7 @@ class TC_GAME_API Creature : public Unit, public GridObject<Creature>, public Ma
         void setDeathState(DeathState s) override;                   // override virtual Unit::setDeathState
 
         bool LoadFromDB(ObjectGuid::LowType spawnId, Map* map) { return LoadCreatureFromDB(spawnId, map, false); }
-        bool LoadCreatureFromDB(ObjectGuid::LowType spawnId, Map* map, bool addToMap = true);
+        bool LoadCreatureFromDB(ObjectGuid::LowType spawnId, Map* map, bool addToMap = true, bool allowDuplicate = false);
         void SaveToDB();
                                                             // overriden in Pet
         virtual void SaveToDB(uint32 mapid, uint32 spawnMask, uint32 phaseMask);