diff --git a/src/main/java/net/minestom/server/command/CommandManager.java b/src/main/java/net/minestom/server/command/CommandManager.java
index 159a27131..69ae41fb6 100644
--- a/src/main/java/net/minestom/server/command/CommandManager.java
+++ b/src/main/java/net/minestom/server/command/CommandManager.java
@@ -84,7 +84,13 @@ public final class CommandManager {
      *
      * @param command the command to register
      */
-    public void register(@NotNull Command command) {
+    public synchronized void register(@NotNull Command command) {
+        Check.stateCondition(commandExists(command.getName()),
+                "A command with the name " + command.getName() + " is already registered!");
+        for (String alias : command.getAliases()) {
+            Check.stateCondition(commandExists(alias),
+                    "A command with the name " + alias + " is already registered!");
+        }
         this.dispatcher.register(command);
     }
 
@@ -104,12 +110,18 @@ public final class CommandManager {
      *
      * @param commandProcessor the command to register
      */
-    public void register(@NotNull CommandProcessor commandProcessor) {
-        this.commandProcessorMap.put(commandProcessor.getCommandName().toLowerCase(), commandProcessor);
+    public synchronized void register(@NotNull CommandProcessor commandProcessor) {
+        final String commandName = commandProcessor.getCommandName().toLowerCase();
+        Check.stateCondition(commandExists(commandName),
+                "A command with the name " + commandName + " is already registered!");
+        this.commandProcessorMap.put(commandName, commandProcessor);
         // Register aliases
         final String[] aliases = commandProcessor.getAliases();
         if (aliases != null && aliases.length > 0) {
             for (String alias : aliases) {
+                Check.stateCondition(commandExists(alias),
+                        "A command with the name " + alias + " is already registered!");
+
                 this.commandProcessorMap.put(alias.toLowerCase(), commandProcessor);
             }
         }
@@ -126,6 +138,18 @@ public final class CommandManager {
         return commandProcessorMap.get(commandName.toLowerCase());
     }
 
+    /**
+     * Gets if a command with the name {@code commandName} already exists or name.
+     *
+     * @param commandName the command name to check
+     * @return true if the command does exist
+     */
+    public boolean commandExists(@NotNull String commandName) {
+        commandName = commandName.toLowerCase();
+        return dispatcher.findCommand(commandName) != null ||
+                commandProcessorMap.get(commandName) != null;
+    }
+
     /**
      * Executes a command for a {@link ConsoleSender}.
      *
diff --git a/src/main/java/net/minestom/server/command/builder/CommandDispatcher.java b/src/main/java/net/minestom/server/command/builder/CommandDispatcher.java
index fb812291a..bf35a3b7e 100644
--- a/src/main/java/net/minestom/server/command/builder/CommandDispatcher.java
+++ b/src/main/java/net/minestom/server/command/builder/CommandDispatcher.java
@@ -17,6 +17,12 @@ public class CommandDispatcher {
     private final Map<String, Command> commandMap = new HashMap<>();
     private final Set<Command> commands = new HashSet<>();
 
+    /**
+     * Registers a command,
+     * be aware that registering a command name or alias will override the previous entry.
+     *
+     * @param command the command to register
+     */
     public void register(@NotNull Command command) {
         this.commandMap.put(command.getName().toLowerCase(), command);
 
diff --git a/src/test/java/demo/Main.java b/src/test/java/demo/Main.java
index ff2cf2a91..99b0e320a 100644
--- a/src/test/java/demo/Main.java
+++ b/src/test/java/demo/Main.java
@@ -32,7 +32,6 @@ public class Main {
         commandManager.register(new EntitySelectorCommand());
         commandManager.register(new HealthCommand());
         commandManager.register(new SimpleCommand());
-        commandManager.register(new GamemodeCommand());
         commandManager.register(new DimensionCommand());
         commandManager.register(new ShutdownCommand());
         commandManager.register(new TeleportCommand());
diff --git a/src/test/java/demo/commands/TestCommand.java b/src/test/java/demo/commands/TestCommand.java
index e99ff2fa9..87bb21c39 100644
--- a/src/test/java/demo/commands/TestCommand.java
+++ b/src/test/java/demo/commands/TestCommand.java
@@ -5,20 +5,14 @@ import net.minestom.server.command.builder.Arguments;
 import net.minestom.server.command.builder.Command;
 import net.minestom.server.command.builder.arguments.Argument;
 import net.minestom.server.command.builder.arguments.ArgumentType;
-import net.minestom.server.utils.location.RelativeVec;
 
 public class TestCommand extends Command {
 
     public TestCommand() {
         super("testcmd");
         setDefaultExecutor(this::usage);
-        {
-            //Argument dynamicWord = ArgumentType.DynamicWord("test");
 
-            //addSyntax(this::execute, dynamicWord);
-        }
-
-        Argument test = ArgumentType.RelativeVec2("pos");
+        Argument test = ArgumentType.ItemStack("test");
 
         test.setCallback((source, value, error) -> {
             System.out.println("ERROR " + error);
@@ -30,8 +24,7 @@ public class TestCommand extends Command {
         });
 
         addSyntax((source, args) -> {
-            RelativeVec location = args.getRelativeVector("pos");
-            System.out.println("IT WORKS " + location.fromRelativePosition(source.asPlayer()));
+            System.out.println("ARG 1");
         }, test);
     }