Paper/paper-server/patches/sources/net/minecraft/server/commands/CommandSummon.patch

--- a/net/minecraft/server/commands/CommandSummon.java
+++ b/net/minecraft/server/commands/CommandSummon.java
@@ -67,7 +67,7 @@
                     ((EntityInsentient) entity).finalizeSpawn(commandlistenerwrapper.getLevel(), commandlistenerwrapper.getLevel().getCurrentDifficultyAt(entity.blockPosition()), EntitySpawnReason.COMMAND, (GroupDataEntity) null);
                 }
 
-                if (!worldserver.tryAddFreshEntityWithPassengers(entity)) {
+                if (!worldserver.tryAddFreshEntityWithPassengers(entity, org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason.COMMAND)) { // CraftBukkit - pass a spawn reason of "COMMAND"
                     throw CommandSummon.ERROR_DUPLICATE_UUID.create();
                 } else {
                     return entity;