diff --git a/lib/cri/command.rb b/lib/cri/command.rb
index be0135c..ba424eb 100644
--- a/lib/cri/command.rb
+++ b/lib/cri/command.rb
@@ -361,7 +361,11 @@ def run_this(opts_and_args, parent_opts = {})
     end
 
     def all_opt_defns
-      supercommand ? supercommand.all_opt_defns.merge(option_definitions) : option_definitions
+      if supercommand
+        supercommand.all_opt_defns | option_definitions
+      else
+        option_definitions
+      end
     end
 
     # @return [String] The help text for this command
diff --git a/test/test_command.rb b/test/test_command.rb
index b2108cb..9f0b1af 100644
--- a/test/test_command.rb
+++ b/test/test_command.rb
@@ -963,5 +963,42 @@ def test_required_option_defaults_to_given_value_with_transform
       assert_equal ['Animal = cow'], lines(out)
       assert_equal [], lines(err)
     end
+
+    def test_option_definitions_are_not_shared_across_commands
+      root_cmd = Cri::Command.define do
+        name 'root'
+        option :r, :rrr, 'Rrr!', argument: :required
+      end
+
+      subcmd_a = root_cmd.define_command do
+        name 'a'
+        option :a, :aaa, 'Aaa!', argument: :required
+
+        run do |_opts, _args, cmd|
+          puts cmd.all_opt_defns.map(&:long).sort.join(',')
+        end
+      end
+
+      subcmd_b = root_cmd.define_command do
+        name 'b'
+        option :b, :bbb, 'Bbb!', argument: :required
+
+        run do |_opts, _args, cmd|
+          puts cmd.all_opt_defns.map(&:long).sort.join(',')
+        end
+      end
+
+      out, err = capture_io_while do
+        subcmd_a.run(%w[])
+      end
+      assert_equal ['aaa,rrr'], lines(out)
+      assert_equal [], lines(err)
+
+      out, err = capture_io_while do
+        subcmd_b.run(%w[])
+      end
+      assert_equal ['bbb,rrr'], lines(out)
+      assert_equal [], lines(err)
+    end
   end
 end