My friends can join my sever but when I join it says internal error. I attached the Crash-report.
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[media]
Description: Exception ticking world
java.lang.RuntimeException: Already decorating
at alg.a(SourceFile:23)
at akz.a(SourceFile:387)
at alv.a(SourceFile:74)
at avt.a(SourceFile:443)
at ny.a(SourceFile:175)
at aup.a(SourceFile:855)
at ny.c(SourceFile:104)
at ny.d(SourceFile:117)
at akh.b(SourceFile:287)
at nz.h(SourceFile:271)
at nz.c(SourceFile:162)
at net.minecraft.server.MinecraftServer.w(SourceFile:539)
at mz.w(SourceFile:272)
at net.minecraft.server.MinecraftServer.v(SourceFile:481)
at net.minecraft.server.MinecraftServer.run(SourceFile:397)
at mq.run(SourceFile:667)
Open the command line and type
java -version
Then hit enter and post the result here.
java version "1.1.0_51"
java(TM) SE Runtime Enviroment (build 1.7.0_51-b13)
Java HotSpot(TM) 64-Bit Server VM (build 24.51-b03, mixed mode)
-Hope This is what you needed

This ticket has been marked as a duplicate of MC-58795, in favor of the more general or better information provided.