diff --git a/rpn.go b/rpn.go
index 15b5656..937f0f9 100644
--- a/rpn.go
+++ b/rpn.go
@@ -160,6 +160,7 @@ DESCRIPTION
         shift                remove the last element of the stack
         reverse              reverse the stack elements
         swap                 exchange the last two stack elements
+        dup                  duplicate last stack item
         history              display calculation history
         help|?               show this message
         quit|exit|c-d|c-c    exit program
diff --git a/rpn.pod b/rpn.pod
index e36e8c8..79a5ffd 100644
--- a/rpn.pod
+++ b/rpn.pod
@@ -167,6 +167,7 @@ Commands:
     shift                remove the last element of the stack
     reverse              reverse the stack elements
     swap                 exchange the last two stack elements
+    dup                  duplicate last stack item
     history              display calculation history
     help|?               show this message
     quit|exit|c-d|c-c    exit program