When trying to force a battle between the player and a single NPC trainer with the Double Battle battle rule type, the game reports "an unknown error occurred while attempting to perform this command" message in chat. Single battle mode on the same NPC works. In the console I receive the message

[14:26:16] [Server thread/WARN] [minecraft/CommandHandler]: Couldn't process command: pokebattle @s MyNameGuy
java.lang.ArrayIndexOutOfBoundsException: 2
at com.pixelmonmod.pixelmon.commands.BattleCommand.execute(BattleCommand.java:114) ~[BattleCommand.class:8.1.2]
at com.pixelmonmod.pixelmon.api.command.PixelmonCommand.func_184881_a(PixelmonCommand.java:87) ~[PixelmonCommand.class:8.1.2]
at net.minecraft.command.CommandHandler.func_175786_a(CommandHandler.java:119) [bj.class:?]
at net.minecraft.command.CommandHandler.func_71556_a(CommandHandler.java:91) [bj.class:?]
at net.minecraft.network.NetHandlerPlayServer.func_147361_d(NetHandlerPlayServer.java:962) [pa.class:?]
at net.minecraft.network.NetHandlerPlayServer.func_147354_a(NetHandlerPlayServer.java:941) [pa.class:?]
at net.minecraft.network.play.client.CPacketChatMessage.func_148833_a(SourceFile:37) [la.class:?]
at net.minecraft.network.play.client.CPacketChatMessage.func_148833_a(SourceFile:9) [la.class:?]
at net.minecraft.network.PacketThreadUtil$1.run(SourceFile:13) [hv$1.class:?]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.8.0_261]
at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_261]
at net.minecraft.util.Util.func_181617_a(SourceFile:46) [h.class:?]
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:723) [MinecraftServer.class:?]
at net.minecraft.server.dedicated.DedicatedServer.func_71190_q(DedicatedServer.java:397) [nz.class:?]
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:668) [MinecraftServer.class:?]
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:526) [MinecraftServer.class:?]
at java.lang.Thread.run(Unknown Source) [?:1.8.0_261]

Thank you for the report. I was able to replicate the issue and get pretty much the same error.

Forwarded to our internal tracker.