error spawns

2013-11-19 19:11:32 [SEVERE] "Silently" catching entity tracking error.
2013-11-19 19:11:32 [SEVERE] net.minecraft.server.v1_6_R2.ReportedException: Adding entity to track
2013-11-19 19:11:32 [SEVERE] 	at net.minecraft.server.v1_6_R2.EntityTracker.addEntity(EntityTracker.java:118)
2013-11-19 19:11:32 [SEVERE] 	at net.minecraft.server.v1_6_R2.EntityTracker.track(EntityTracker.java:69)
2013-11-19 19:11:32 [SEVERE] 	at net.minecraft.server.v1_6_R2.WorldManager.a(WorldManager.java:18)
2013-11-19 19:11:32 [SEVERE] 	at net.minecraft.server.v1_6_R2.World.a(World.java:1025)
2013-11-19 19:11:32 [SEVERE] 	at net.minecraft.server.v1_6_R2.WorldServer.a(WorldServer.java:793)
2013-11-19 19:11:32 [SEVERE] 	at net.minecraft.server.v1_6_R2.World.addEntity(World.java:1018)
2013-11-19 19:11:32 [SEVERE] 	at eu.sylian.spawns.Mob$1.run(Mob.java:246)
2013-11-19 19:11:32 [SEVERE] 	at org.bukkit.craftbukkit.v1_6_R2.scheduler.CraftTask.run(CraftTask.java:67)
2013-11-19 19:11:32 [SEVERE] 	at org.bukkit.craftbukkit.v1_6_R2.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:345)
2013-11-19 19:11:32 [SEVERE] 	at net.minecraft.server.v1_6_R2.MinecraftServer.t(MinecraftServer.java:518)
2013-11-19 19:11:32 [SEVERE] 	at net.minecraft.server.v1_6_R2.DedicatedServer.t(DedicatedServer.java:239)
2013-11-19 19:11:32 [SEVERE] 	at net.minecraft.server.v1_6_R2.MinecraftServer.s(MinecraftServer.java:481)
2013-11-19 19:11:32 [SEVERE] 	at net.minecraft.server.v1_6_R2.MinecraftServer.run(MinecraftServer.java:413)
2013-11-19 19:11:32 [SEVERE] 	at net.minecraft.server.v1_6_R2.ThreadServerApplication.run(SourceFile:582)
2013-11-19 19:11:32 [SEVERE] Caused by: java.lang.IllegalStateException: Entity is already tracked!
2013-11-19 19:11:32 [SEVERE] 	at net.minecraft.server.v1_6_R2.EntityTracker.addEntity(EntityTracker.java:98)
2013-11-19 19:11:32 [SEVERE] 	... 13 more