Le problème semble venir de cofh core :
Caused by: java.lang.ClassFormatError: Duplicate method name&signature in class file net/minecraft/block/BlockLeavesBase
at java.lang.ClassLoader.defineClass1(Native Method) ~[?:1.8.0_121]
at java.lang.ClassLoader.defineClass(Unknown Source) ~[?:1.8.0_121]
at java.security.SecureClassLoader.defineClass(Unknown Source) ~[?:1.8.0_121]
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:182) ~[launchwrapper-1.12.jar:?]
at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_121]
at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_121]
at java.lang.Class.forName0(Native Method) ~[?:1.8.0_121]
at java.lang.Class.forName(Unknown Source) ~[?:1.8.0_121]
at cofh.asm.CoFHAccessTransformer.transform(CoFHAccessTransformer.java:90) ~[CoFHCore-%5B1.7.10%5D3.1.2-325.jar:?]
at net.minecraft.launchwrapper.LaunchClassLoader.runTransformers(LaunchClassLoader.java:279) ~[launchwrapper-1.12.jar:?]
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:176) ~[launchwrapper-1.12.jar:?]
at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_121]
at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_121]
at java.lang.Class.forName0(Native Method) ~[?:1.8.0_121]
at java.lang.Class.forName(Unknown Source) ~[?:1.8.0_121]
at cofh.asm.CoFHAccessTransformer.transform(CoFHAccessTransformer.java:90) ~[CoFHCore-%5B1.7.10%5D3.1.2-325.jar:?]
at net.minecraft.launchwrapper.LaunchClassLoader.runTransformers(LaunchClassLoader.java:279) ~[launchwrapper-1.12.jar:?]
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:176) ~[launchwrapper-1.12.jar:?]
at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_121]
at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0_121]
at net.minecraft.block.Block.func_149671_p(Block.java:222) ~[aji.class:?]
at net.minecraft.init.Bootstrap.func_151354_b(SourceFile:355) ~[kl.class:?]
at net.minecraft.client.Minecraft.<init>(Minecraft.java:287) ~[bao.class:?]
at net.minecraft.client.main.Main.main(SourceFile:129) ~[Main.class:?]
... 14 more