Fix #12721 (#12725)

* Fix #12721

Track when knots are created and only fire the event then

* Use mutable boolean + check if actually spawned

paper-server/patches/sources/net/minecraft/world/entity/decoration/LeashFenceKnotEntity.java.patch

@@ -27,3 +27,25 @@
          }
      }
  
+@@ -120,7 +_,12 @@
+         return this.level().getBlockState(this.pos).is(BlockTags.FENCES);
+     }
+ 
++    // Paper start - Track if a knot was created
+     public static LeashFenceKnotEntity getOrCreateKnot(Level level, BlockPos pos) {
++        return getOrCreateKnot(level, pos, null);
++    }
++    public static LeashFenceKnotEntity getOrCreateKnot(Level level, BlockPos pos, @Nullable org.apache.commons.lang3.mutable.MutableBoolean created) {
++        // Paper end - Track if a knot was created
+         int x = pos.getX();
+         int y = pos.getY();
+         int z = pos.getZ();
+@@ -134,7 +_,7 @@
+         }
+ 
+         LeashFenceKnotEntity leashFenceKnotEntity1 = new LeashFenceKnotEntity(level, pos);
+-        level.addFreshEntity(leashFenceKnotEntity1);
++        if (level.addFreshEntity(leashFenceKnotEntity1) && created != null) { created.setTrue(); } // Paper - Track if a knot was created
+         return leashFenceKnotEntity1;
+     }
+