Check for duplicate entries in active_npc before inserting.

src/game.cpp

@@ -869,6 +869,14 @@ void game::load_npcs()
     // uses submap coordinates
     std::vector<std::shared_ptr<npc>> just_added;
     for( const auto &temp : overmap_buffer.get_npcs_near_player( radius ) ) {
+        const character_id &id = temp->getID();
+        const auto found = std::find_if( active_npc.begin(), active_npc.end(),
+        [id]( const std::shared_ptr<npc> &n ) {
+            return n->getID() == id;
+        } );
+        if( found != active_npc.end() ) {
+            continue;
+        }
         if( temp->is_active() ) {
             continue;
         }

src/game.h

@@ -1011,7 +1011,7 @@ class game
         bool safe_mode_warning_logged;
         bool bVMonsterLookFire;
         character_id next_npc_id;
-        std::vector<std::shared_ptr<npc>> active_npc;
+        std::list<std::shared_ptr<npc>> active_npc;
         int next_mission_id;
         std::set<character_id> follower_ids; // Keep track of follower NPC IDs
         int moves_since_last_save;

src/overmapbuffer.cpp

@@ -1003,7 +1003,7 @@ void overmapbuffer::insert_npc( const std::shared_ptr<npc> &who )
     get( npc_om_pos ).insert_npc( who );
 }
 
-std::shared_ptr<npc> overmapbuffer::remove_npc( const character_id id )
+std::shared_ptr<npc> overmapbuffer::remove_npc( const character_id &id )
 {
     for( auto &it : overmaps ) {
         if( const auto p = it.second->erase_npc( id ) ) {

src/overmapbuffer.h

@@ -255,7 +255,7 @@ class overmapbuffer
          * Find npc by id and if found, erase it from the npc list
          * and return it ( or return nullptr if not found ).
          */
-        std::shared_ptr<npc> remove_npc( character_id id );
+        std::shared_ptr<npc> remove_npc( const character_id &id );
         /**
          * Adds the npc to an overmap ( based on the npcs current location )
          * and stores it there. The overmap takes ownership of the pointer.