What does it mean to duplicate a stack?
I'm reading **The C Programming Language** and learned how to make a reverse Polish calculator using a stack. Here is one of the exercises that follow it:

>**Exercise 4-4.** Add the commands to print the top elements of the stack without popping, to duplicate it, and to swap the top
two elements. Add a command to clear the stack.

What do they mean by "duplicate"? Does it mean to print out the entire stack, or to push the entire stack onto itself (so that, for example, "1 2 3" would become "1 2 3 1 2 3"), or what?
What does it mean to duplicate a stack?
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I'm reading **The C Programming Language** and learned how to make a reverse Polish calculator using a stack. Here is one of the exercises that follow it:

>**Exercise 4-4.** Add the commands to print the top elements of the stack without popping, to duplicate it, and to swap the top
two elements. Add a command to clear the stack.

What do they mean by "duplicate"? Does it mean to print out the entire stack, or to push the entire stack onto itself (so that, for example, "1 2 3" would become "1 2 3 1 2 3"), or what?