=proto
void duk_dup(duk_context *ctx, int from_index);

=stack
[ ... val! ... ] -> [ ... val! ... val! ]

=summary
<p>Push a duplicate of value at <tt>from_index</tt> to the stack.
If <tt>from_index</tt> is invalid, throws an error.</p>

=example
duk_push_int(ctx, 123);  /* -> [ ... 123 ] */
duk_push_int(ctx, 234);  /* -> [ ... 123 234 ] */
duk_dup(ctx, -2);        /* -> [ ... 123 234 123 ] */