[15:44:02] [Server thread/ERROR]: "Silently" catching entity tracking error.
net.minecraft.util.ReportedException: Adding entity to track
        at net.minecraft.entity.EntityTracker.func_72785_a(EntityTracker.java:246) ~[lp.class:?]
        at net.minecraft.entity.EntityTracker.func_72786_a(EntityTracker.java:171) ~[lp.class:?]
        at net.minecraft.world.ServerWorldEventHandler.func_72703_a(ServerWorldEventHandler.java:33) ~[lt.class:?]
        at net.minecraft.world.World.func_72923_a(World.java:1142) ~[aid.class:?]
        at net.minecraft.world.WorldServer.func_72923_a(WorldServer.java:1117) ~[ls.class:?]
        at net.minecraft.world.WorldServer.onSpongeEntityAdded(WorldServer.java:1374) ~[ls.class:?]
        at net.minecraft.world.WorldServer.forceSpawnEntity(WorldServer.java:1420) ~[ls.class:?]
        at net.minecraft.world.WorldServer.forceSpawnEntity(WorldServer.java:1403) ~[ls.class:?]
        at org.spongepowered.common.event.tracking.CauseTracker.spawnEntityWithCause(CauseTracker.java:582) ~[CauseTracker.class:1.10.2-2107-6.0.0-BETA-1864]
        at net.minecraft.world.WorldServer.spawnEntity(WorldServer.java:1391) ~[ls.class:?]
        at com.gmail.trentech.mobspawners.Main.lambda$spawn$0(Main.java:205) ~[Main.class:?]
        at org.spongepowered.common.scheduler.SchedulerBase.lambda$startTask$0(SchedulerBase.java:177) ~[SchedulerBase.class:1.10.2-2107-6.0.0-BETA-1864]
        at org.spongepowered.common.scheduler.SyncScheduler.executeTaskRunnable(SyncScheduler.java:66) ~[SyncScheduler.class:1.10.2-2107-6.0.0-BETA-1864]
        at org.spongepowered.common.scheduler.SchedulerBase.startTask(SchedulerBase.java:174) ~[SchedulerBase.class:1.10.2-2107-6.0.0-BETA-1864]
        at org.spongepowered.common.scheduler.SchedulerBase.processTask(SchedulerBase.java:160) ~[SchedulerBase.class:1.10.2-2107-6.0.0-BETA-1864]
        at java.util.concurrent.ConcurrentHashMap$ValuesView.forEach(ConcurrentHashMap.java:4707) [?:1.8.0_111]
        at org.spongepowered.common.scheduler.SchedulerBase.runTick(SchedulerBase.java:104) [SchedulerBase.class:1.10.2-2107-6.0.0-BETA-1864]
        at org.spongepowered.common.scheduler.SyncScheduler.tick(SyncScheduler.java:41) [SyncScheduler.class:1.10.2-2107-6.0.0-BETA-1864]
        at org.spongepowered.common.scheduler.SpongeScheduler.tickSyncScheduler(SpongeScheduler.java:191) [SpongeScheduler.class:1.10.2-2107-6.0.0-BETA-1864]
        at org.spongepowered.mod.SpongeMod.onTick(SpongeMod.java:258) [SpongeMod.class:1.10.2-2107-6.0.0-BETA-1864]
        at net.minecraftforge.fml.common.eventhandler.ASMEventHandler_10_SpongeMod_onTick_ServerTickEvent.invoke(.dynamic) [?:?]
        at net.minecraftforge.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:90) [ASMEventHandler.class:?]
        at net.minecraftforge.fml.common.eventhandler.EventBus.post(EventBus.java:130) [EventBus.class:?]
        at net.minecraftforge.fml.common.eventhandler.EventBus.post(EventBus.java:99) [EventBus.class:?]
        at net.minecraftforge.fml.common.FMLCommonHandler.onPreServerTick(FMLCommonHandler.java:274) [FMLCommonHandler.class:?]
        at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:602) [MinecraftServer.class:?]
        at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:471) [MinecraftServer.class:?]
        at java.lang.Thread.run(Thread.java:745) [?:1.8.0_111]
Caused by: java.lang.IllegalStateException: Entity EntitySkeleton['Skeleton'/909, l='world', x=603.46, y=73.00, z=1423.65] is already tracked for world: world
        at net.minecraft.entity.EntityTracker.redirect$reportEntityAlreadyTrackedWithWorld$zda000(EntityTracker.java:65) ~[lp.class:?]
        at net.minecraft.entity.EntityTracker.func_72785_a(EntityTracker.java:214) ~[lp.class:?]
        ... 27 more