More idiomatic (and complete) include guards.

a_actor.inc

@@ -4,9 +4,10 @@
  *
  */
 
-#if defined _actor_included
+#if defined _INC_a_actor
 	#endinput
 #endif
+#define _INC_a_actor
 #define _actor_included
 
 /// <summary>Create a static 'actor' in the world. These 'actors' are like NPCs, however they have limited functionality. They do not take up server player slots.</summary>

a_http.inc

@@ -4,6 +4,11 @@
  *
  */
 
+#if defined _INC_a_http
+	#endinput
+#endif
+#define _INC_a_http
+
 // HTTP requests
 #define HTTP_GET						1
 #define HTTP_POST						2

a_npc.inc

@@ -4,9 +4,10 @@
  *
  */
 
-#if defined _samp_included
+#if defined _INC_a_npc
 	#endinput
 #endif
+#define _INC_a_npc
 #define _samp_included
 
 #pragma tabsize 4

a_objects.inc

@@ -4,9 +4,10 @@
  *
  */
 
-#if defined _objects_included
+#if defined _INC_a_objects
 	#endinput
 #endif
+#define _INC_a_objects
 #define _objects_included
 
 // Objects

a_players.inc

@@ -4,9 +4,10 @@
  *
  */
 
-#if defined _players_included
+#if defined _INC_a_players
 	#endinput
 #endif
+#define _INC_a_players
 #define _players_included
 
 #define SPECIAL_ACTION_NONE				0

a_samp.inc

@@ -4,9 +4,10 @@
  *
  */
 
-#if defined _samp_included
+#if defined _INC_a_samp
 	#endinput
 #endif
+#define _INC_a_samp
 #define _samp_included
 
 #pragma tabsize 4

a_sampdb.inc

@@ -3,9 +3,10 @@
 //  (c) Copyright 2015, SA-MP Team
 //
 
-#if defined _sampdb_included
+#if defined _INC_a_sampdb
 	#endinput
 #endif
+#define _INC_a_sampdb
 #define _sampdb_included
 
 /// <summary>This function is used to open a connection to a SQLite database, which is inside the <b><c>/scriptfiles</c></b> folder.</summary>

a_vehicles.inc

@@ -4,9 +4,10 @@
  *
  */
 
-#if defined _vehicles_included
+#if defined _INC_a_vehicles
 	#endinput
 #endif
+#define _INC_a_vehicles
 #define _vehicles_included
 
 #define CARMODTYPE_SPOILER		0