✨ Prevent command arguments from being used in multiple commands

Also get rid of some random annoying warnings.

cloud-core/src/main/java/cloud/commandframework/Command.java

@@ -384,6 +384,11 @@ public class Command<C> {
          */
         public <T> @NonNull Builder<C> argument(final @NonNull CommandArgument<C, T> argument,
                                                 final @NonNull Description description) {
+            if (argument.isArgumentRegistered()) {
+                throw new IllegalArgumentException("The provided argument has already been associated with a command."
+                                                 + " Use CommandArgument#copy to create a copy of the argument.");
+            }
+            argument.setArgumentRegistered();
             final Map<CommandArgument<C, ?>, Description> commandArgumentMap = new LinkedHashMap<>(this.commandArguments);
             commandArgumentMap.put(argument, description);
             return new Builder<>(this.commandManager,

cloud-core/src/main/java/cloud/commandframework/arguments/CommandArgument.java

@@ -82,6 +82,10 @@ public class CommandArgument<C, T> implements Comparable<CommandArgument<?, ?>>
      * Suggestion provider
      */
     private final BiFunction<CommandContext<C>, String, List<String>> suggestionsProvider;
+    /**
+     * Whether or not the argument has been used before
+     */
+    private boolean argumentRegistered = false;
 
     private Command<C> owningCommand;
 
@@ -328,6 +332,22 @@ public class CommandArgument<C, T> implements Comparable<CommandArgument<?, ?>>
         return builder.build();
     }
 
+    /**
+     * Check whether or not the argument has been used in a command
+     *
+     * @return {@code true} if the argument has been used in a command, else {@code false}
+     */
+    public boolean isArgumentRegistered() {
+        return this.argumentRegistered;
+    }
+
+    /**
+     * Indicate that the argument has been associated with a command
+     */
+    public void setArgumentRegistered() {
+        this.argumentRegistered = true;
+    }
+
 
     /**
      * Mutable builder for {@link CommandArgument} instances

cloud-core/src/test/java/cloud/commandframework/CommandTreeTest.java

@@ -23,6 +23,7 @@
 //
 package cloud.commandframework;
 
+import cloud.commandframework.arguments.CommandArgument;
 import cloud.commandframework.arguments.compound.ArgumentPair;
 import cloud.commandframework.arguments.standard.IntegerArgument;
 import cloud.commandframework.arguments.standard.StringArgument;
@@ -213,6 +214,15 @@ class CommandTreeTest {
         manager.executeCommand(new TestCommandSender(), "flags --num 500");
     }
 
+    @Test
+    void testDuplicateArgument() {
+        final CommandArgument<TestCommandSender, String> argument = StringArgument.of("test");
+        manager.command(manager.commandBuilder("one").argument(argument));
+        Assertions.assertThrows(IllegalArgumentException.class, () ->
+                manager.command(manager.commandBuilder("two").argument(argument)));
+    }
+
+
     public static final class SpecificCommandSender extends TestCommandSender {
     }