diff --git a/lib/thor.rb b/lib/thor.rb
index cf999e6e..d20f332a 100644
--- a/lib/thor.rb
+++ b/lib/thor.rb
@@ -327,6 +327,8 @@ def subcommand_classes
     end
 
     def subcommand(subcommand, subcommand_class)
+      return subcommand_classes[subcommand.to_s] if subcommands.include? subcommand.to_s
+
       subcommands << subcommand.to_s
       subcommand_class.subcommand_help subcommand
       subcommand_classes[subcommand.to_s] = subcommand_class