Startup error

[SEVERE] Error occurred while enabling GroupManager v1.7 (Phoenix) (Is it up to date?): I think this user was declared more than once: g:Guest in file: plugins\GroupManager\worlds\world\groups.yml
java.lang.IllegalArgumentException: I think this user was declared more than once: g:Guest in file: plugins\GroupManager\worlds\world\groups.yml
	at org.anjocaido.groupmanager.dataholder.WorldDataHolder.loadGroups(WorldDataHolder.java:616)
	at org.anjocaido.groupmanager.dataholder.WorldDataHolder.load(WorldDataHolder.java:571)
	at org.anjocaido.groupmanager.dataholder.worlds.WorldsHolder.loadWorld(WorldsHolder.java:460)
	at org.anjocaido.groupmanager.dataholder.worlds.WorldsHolder.initialWorldLoading(WorldsHolder.java:75)
	at org.anjocaido.groupmanager.dataholder.worlds.WorldsHolder.initialLoad(WorldsHolder.java:66)
	at org.anjocaido.groupmanager.dataholder.worlds.WorldsHolder.<init>(WorldsHolder.java:58)
	at org.anjocaido.groupmanager.GroupManager.onEnable(GroupManager.java:131)
	at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:188)
	at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader.java:968)
	at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:280)
	at org.bukkit.craftbukkit.CraftServer.loadPlugin(CraftServer.java:186)
	at org.bukkit.craftbukkit.CraftServer.enablePlugins(CraftServer.java:169)
	at net.minecraft.server.MinecraftServer.t(MinecraftServer.java:348)
	at net.minecraft.server.MinecraftServer.a(MinecraftServer.java:335)
	at net.minecraft.server.MinecraftServer.init(MinecraftServer.java:165)
	at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:399)
	at net.minecraft.server.ThreadServerApplication.run(SourceFile:457)