diff --git a/src/main/java/co/aikar/commands/RootCommand.java b/src/main/java/co/aikar/commands/RootCommand.java
index 7cb3aac4..4e91aac2 100644
--- a/src/main/java/co/aikar/commands/RootCommand.java
+++ b/src/main/java/co/aikar/commands/RootCommand.java
@@ -24,6 +24,7 @@
 package co.aikar.commands;
 
 import org.apache.commons.lang.StringUtils;
+import org.bukkit.Bukkit;
 import org.bukkit.command.Command;
 import org.bukkit.command.CommandSender;
 
@@ -76,7 +77,15 @@ public class RootCommand extends Command {
         if (this.defCommand == null || command.subCommands.get("__default") != null) {
             this.defCommand = command;
         }
-        command.subCommands.keySet().forEach(key -> this.subCommands.put(key, command));
+        command.subCommands.keySet().forEach(key -> {
+            BaseCommand regged = this.subCommands.get(key);
+            if (regged != null) {
+                ACFLog.severe("ACF Error: " + command.getLabel() + " registered subcommand " + key + " - but it is already defined in " + regged.getLabel());
+                ACFLog.severe("2 subcommands of the same prefix may not be spread over 2 different classes. Ignoring this.");
+                return;
+            }
+            this.subCommands.put(key, command);
+        });
         this.children.add(command);
     }
 }