Get an error can someone help me :(

---- Minecraft Crash Report ----
// Everything's going to plan. No, really, that was supposed to happen.

Time: 25-4-13 23:51
Description: Exception in server tick loop

java.lang.NoSuchMethodError: com.google.common.collect.Multisets.copyHighestCountFirst(Lcom/google/common/collect/Multiset;)Lcom/google/common/collect/ImmutableMultiset;
at cpw.mods.fml.common.Loader.identifyDuplicates(Loader.java:378)
at cpw.mods.fml.common.Loader.identifyMods(Loader.java:347)
at cpw.mods.fml.common.Loader.loadMods(Loader.java:478)
at cpw.mods.fml.server.FMLServerHandler.beginServerLoading(FMLServerHandler.java:85)
at cpw.mods.fml.common.FMLCommonHandler.onServerStart(FMLCommonHandler.java:352)
at net.minecraft.server.dedicated.DedicatedServer.func_71197_b(DedicatedServer.java:69)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:429)
at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:573)


A detailed walkthrough of the error, its code path and all known details is as follows:
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-- System Details --
Details:
Minecraft Version: 1.5.1
Operating System: Windows 7 (x86) version 6.1
Java Version: 1.7.0_21, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
Memory: 947689912 bytes (903 MB) / 1037959168 bytes (989 MB) up to 1037959168 bytes (989 MB)
JVM Flags: 2 total; -Xms1024M -Xmx1024M
AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
Suspicious classes: FML and Forge are installed
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
FML: MCP v7.44 FML v5.1.8.611 Minecraft Forge 7.7.1.611 6 mods loaded, 0 mods active
mcp [Minecraft Coder Pack] (minecraft.jar)
FML [Forge Mod Loader] (coremods)
Forge [Minecraft Forge] (coremods)
BukkitForge [BukkitForge] (craftbukkit-1.5-R0.1-20130315.192915-2.jar)
BlockBreak [Block Break Event] (craftbukkit-1.5-R0.1-20130315.192915-2.jar)
Pixelmon [Pixelmon] (Pixelmon 2.1.2.zip)
Profiler Position: N/A (disabled)
Is Modded: Definitely; Server brand changed to 'fml'
Type: Dedicated Server (map_server.txt)