Hopper: java.lang.IllegalArgumentException: Modifier is already applied on this attribute!

This is an automatically generated report based from the following crash: http://hopper.minecraft.net/crashes/minecraft/MCX-2555321/

Crash:

java.lang.IllegalArgumentException: Modifier is already applied on this attribute!
	at ry.b(SourceFile:89)
	at xx.c(SourceFile:123)
	at ul.b(SourceFile:82)
	at xx$b.b(SourceFile:430)
	at su.a(SourceFile:135)
	at su.a(SourceFile:107)
	at rl.bT(SourceFile:547)
	at rk.n(SourceFile:1602)
	at rl.n(SourceFile:404)
	at ye.n(SourceFile:33)
	at xx.n(SourceFile:211)
	at rk.m(SourceFile:1470)
	at rl.m(SourceFile:243)
	at ye.m(SourceFile:38)
	at agq.a(SourceFile:1436)
	at ll.a(SourceFile:661)
	at agq.g(SourceFile:1414)

The following is the first clean report made public about this crash:

---- Minecraft Crash Report ----
// Uh... Did I do that?

Time: 9/2/15 5:46 PM
Description: Ticking entity

java.lang.IllegalArgumentException: Modifier is already applied on this attribute!
	at ry.b(SourceFile:89)
	at xx.c(SourceFile:123)
	at ul.b(SourceFile:82)
	at xx$b.b(SourceFile:430)
	at su.a(SourceFile:135)
	at su.a(SourceFile:107)
	at rl.bT(SourceFile:547)
	at rk.n(SourceFile:1602)
	at rl.n(SourceFile:404)
	at ye.n(SourceFile:33)
	at xx.n(SourceFile:211)
	at rk.m(SourceFile:1470)
	at rl.m(SourceFile:243)
	at ye.m(SourceFile:38)
	at agq.a(SourceFile:1436)
	at ll.a(SourceFile:661)
	at agq.g(SourceFile:1414)
	at agq.k(SourceFile:1305)
	at ll.k(SourceFile:540)
	at net.minecraft.server.MinecraftServer.D(SourceFile:625)
	at net.minecraft.server.MinecraftServer.C(SourceFile:553)
	at bvd.C(SourceFile:154)
	at net.minecraft.server.MinecraftServer.run(SourceFile:457)
	at java.lang.Thread.run(Thread.java:695)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Stacktrace:
	at ry.b(SourceFile:89)
	at xx.c(SourceFile:123)
	at ul.b(SourceFile:82)
	at xx$b.b(SourceFile:430)
	at su.a(SourceFile:135)
	at su.a(SourceFile:107)
	at rl.bT(SourceFile:547)
	at rk.n(SourceFile:1602)
	at rl.n(SourceFile:404)
	at ye.n(SourceFile:33)
	at xx.n(SourceFile:211)
	at rk.m(SourceFile:1470)
	at rl.m(SourceFile:243)
	at ye.m(SourceFile:38)
	at agq.a(SourceFile:1436)
	at ll.a(SourceFile:661)
	at agq.g(SourceFile:1414)

-- Entity being ticked --
Details:
	Entity Type: Enderman (xx)
	Entity ID: 247069
	Entity Name: Enderman
	Entity's Exact location: -359.32, 4.00, -1120.29
	Entity's Block location: -360.00,4.00,-1121.00 - World: (-360,4,-1121), Chunk: (at 8,0,15 in -23,-71; contains blocks -368,0,-1136 to -353,255,-1121), Region: (-1,-3; contains chunks -32,-96 to -1,-65, blocks -512,0,-1536 to -1,255,-1025)
	Entity's Momentum: 0.00, -0.08, 0.00
	Entity's Rider: ~~ERROR~~ NullPointerException: null
	Entity's Vehicle: ~~ERROR~~ NullPointerException: null
Stacktrace:
	at agq.k(SourceFile:1305)
	at ll.k(SourceFile:540)

-- Affected level --
Details:
	Level name: Snapshot
	All players: 1 total; [ln['OrphanRacer'/123, l='Snapshot', x=-348.49, y=4.00, z=-1118.67]]
	Chunk stats: ServerChunkCache: 1681 Drop: 0
	Level seed: -3198650440654901931
	Level generator: ID 01 - flat, ver 0. Features enabled: false
	Level generator options: 
	Level spawn location: -359.00,4.00,-1116.00 - World: (-359,4,-1116), Chunk: (at 9,0,4 in -23,-70; contains blocks -368,0,-1120 to -353,255,-1105), Region: (-1,-3; contains chunks -32,-96 to -1,-65, blocks -512,0,-1536 to -1,255,-1025)
	Level time: 31011 game time, 13000 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 74538 (now: false), thunder time: 47110 (now: false)
	Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: true
Stacktrace:
	at net.minecraft.server.MinecraftServer.D(SourceFile:625)
	at net.minecraft.server.MinecraftServer.C(SourceFile:553)
	at bvd.C(SourceFile:154)
	at net.minecraft.server.MinecraftServer.run(SourceFile:457)
	at java.lang.Thread.run(Thread.java:695)

-- System Details --
Details:
	Minecraft Version: 15w36b
	Operating System: Mac OS X (x86_64) version 10.11
	Java Version: 1.6.0_65, Apple Inc.
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
	Memory: 146928440 bytes (140 MB) / 379068416 bytes (361 MB) up to 1060372480 bytes (1011 MB)
	JVM Flags: 5 total; -Xmx1G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
	IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
	Profiler Position: N/A (disabled)
	Player Count: 1 / 8; [ln['OrphanRacer'/123, l='Snapshot', x=-348.49, y=4.00, z=-1118.67]]
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.

You can see this full report at http://hopper.minecraft.net/crashes/minecraft/MCX-2555321/74923043/