diff --git a/sources/advanced/Cli.ts b/sources/advanced/Cli.ts
index ffe02d8..03b4e04 100644
--- a/sources/advanced/Cli.ts
+++ b/sources/advanced/Cli.ts
@@ -397,6 +397,10 @@ export class Cli<Context extends BaseContext = BaseContext> implements Omit<Mini
    * Registers a command inside the CLI.
    */
   register(commandClass: CommandClass<Context>) {
+    if (this.registrations.has(commandClass)) {
+      throw new RangeError(`${commandClass.name} has already been registered`);
+    }
+
     const specs = new Map<string, CommandOption<any>>();
 
     const command = new commandClass();